Fix unsealed classes

Core/Controls/FlatButton.cs

@@ -2,7 +2,7 @@
 using System.Windows.Forms;
 
 namespace TweetDuck.Core.Controls{
-    class FlatButton : Button{
+    sealed class FlatButton : Button{
         protected override bool ShowFocusCues => false;
 
         public FlatButton(){

Core/Handling/ContextMenuBrowser.cs

@@ -5,7 +5,7 @@
 using TweetDuck.Core.Utils;
 
 namespace TweetDuck.Core.Handling{
-    class ContextMenuBrowser : ContextMenuBase{
+    sealed class ContextMenuBrowser : ContextMenuBase{
         private const CefMenuCommand MenuGlobal   = (CefMenuCommand)26600;
         private const CefMenuCommand MenuMute     = (CefMenuCommand)26601;
         private const CefMenuCommand MenuSettings = (CefMenuCommand)26602;

Core/Handling/ContextMenuNotification.cs

@@ -3,7 +3,7 @@
 using TweetDuck.Core.Notification;
 
 namespace TweetDuck.Core.Handling{
-    class ContextMenuNotification : ContextMenuBase{
+    sealed class ContextMenuNotification : ContextMenuBase{
         private const CefMenuCommand MenuViewDetail         = (CefMenuCommand)26600;
         private const CefMenuCommand MenuSkipTweet          = (CefMenuCommand)26601;
         private const CefMenuCommand MenuFreeze             = (CefMenuCommand)26602;

Core/Handling/General/BrowserProcessHandler.cs

@@ -2,7 +2,7 @@
 using CefSharp;
 
 namespace TweetDuck.Core.Handling.General{
-    class BrowserProcessHandler : IBrowserProcessHandler{
+    sealed class BrowserProcessHandler : IBrowserProcessHandler{
         void IBrowserProcessHandler.OnContextInitialized(){
             using(IRequestContext ctx = Cef.GetGlobalRequestContext()){
                 ctx.SetPreference("browser.enable_spellchecking", Program.UserConfig.EnableSpellCheck, out string _);

Core/Handling/General/JavaScriptDialogHandler.cs

@@ -7,7 +7,7 @@
 using TweetDuck.Core.Utils;
 
 namespace TweetDuck.Core.Handling.General{
-    class JavaScriptDialogHandler : IJsDialogHandler{
+    sealed class JavaScriptDialogHandler : IJsDialogHandler{
         bool IJsDialogHandler.OnJSDialog(IWebBrowser browserControl, IBrowser browser, string originUrl, CefJsDialogType dialogType, string messageText, string defaultPromptText, IJsDialogCallback callback, ref bool suppressMessage){
             ((ChromiumWebBrowser)browserControl).InvokeSafe(() => {
                 FormMessage form;

Core/Handling/General/LifeSpanHandler.cs

@@ -2,7 +2,7 @@
 using TweetDuck.Core.Utils;
 
 namespace TweetDuck.Core.Handling.General{
-    class LifeSpanHandler : ILifeSpanHandler{
+    sealed class LifeSpanHandler : ILifeSpanHandler{
         public bool OnBeforePopup(IWebBrowser browserControl, IBrowser browser, IFrame frame, string targetUrl, string targetFrameName, WindowOpenDisposition targetDisposition, bool userGesture, IPopupFeatures popupFeatures, IWindowInfo windowInfo, IBrowserSettings browserSettings, ref bool noJavascriptAccess, out IWebBrowser newBrowser){
             newBrowser = null;
 

Core/Handling/RequestHandlerBrowser.cs

@@ -2,7 +2,7 @@
 using TweetDuck.Core.Handling.General;
 
 namespace TweetDuck.Core.Handling{
-    class RequestHandlerBrowser : RequestHandlerBase{
+    sealed class RequestHandlerBrowser : RequestHandlerBase{
         public override void OnRenderProcessTerminated(IWebBrowser browserControl, IBrowser browser, CefTerminationStatus status){
             browser.Reload();
         }

Core/Handling/ResourceHandlerNotification.cs

@@ -4,7 +4,7 @@
 using System.Text;
 
 namespace TweetDuck.Core.Handling{
-    class ResourceHandlerNotification : IResourceHandler{
+    sealed class ResourceHandlerNotification : IResourceHandler{
         private readonly NameValueCollection headers = new NameValueCollection(0);
         private MemoryStream dataIn;
 

Core/Other/FormSettings.cs

@@ -139,7 +139,7 @@ private void control_MouseWheel(object sender, MouseEventArgs e){
             panelContents.Focus();
         }
 
-        private class SettingsTab{
+        private sealed class SettingsTab{
             public Button Button { get; }
 
             public BaseTabSettings Control{

Core/Other/Settings/Export/ExportManager.cs

@@ -176,7 +176,7 @@ private static IEnumerable<PathInfo> EnumerateFilesRelative(string root){
             }) : Enumerable.Empty<PathInfo>();
         }
 
-        private class PathInfo{
+        private sealed class PathInfo{
             public string Full { get; set; }
             public string Relative { get; set; }
         }

Core/Other/Settings/TabSettingsAdvanced.cs

@@ -7,7 +7,7 @@
 using TweetDuck.Core.Utils;
 
 namespace TweetDuck.Core.Other.Settings{
-    partial class TabSettingsAdvanced : BaseTabSettings{
+    sealed partial class TabSettingsAdvanced : BaseTabSettings{
         private static SystemConfig SysConfig => Program.SystemConfig;
 
         private readonly Action<string> reinjectBrowserCSS;

Core/Other/Settings/TabSettingsGeneral.cs

@@ -3,7 +3,7 @@
 using TweetDuck.Updates;
 
 namespace TweetDuck.Core.Other.Settings{
-    partial class TabSettingsGeneral : BaseTabSettings{
+    sealed partial class TabSettingsGeneral : BaseTabSettings{
         private readonly UpdateHandler updates;
         private int updateCheckEventId = -1;
 

Core/Other/Settings/TabSettingsNotifications.cs

@@ -4,7 +4,7 @@
 using TweetDuck.Core.Notification;
 
 namespace TweetDuck.Core.Other.Settings{
-    partial class TabSettingsNotifications : BaseTabSettings{
+    sealed partial class TabSettingsNotifications : BaseTabSettings{
         private static readonly int[] IdlePauseSeconds = { 0, 30, 60, 120, 300 };
 
         private readonly FormNotificationMain notification;

Core/Other/Settings/TabSettingsSounds.cs

@@ -7,7 +7,7 @@
 using TweetLib.Audio;
 
 namespace TweetDuck.Core.Other.Settings{
-    partial class TabSettingsSounds : BaseTabSettings{
+    sealed partial class TabSettingsSounds : BaseTabSettings{
         private readonly SoundNotification soundNotification;
         private readonly bool supportsChangingVolume;
 

Core/TrayIcon.cs

@@ -3,7 +3,7 @@
 using System.Windows.Forms;
 
 namespace TweetDuck.Core{
-    partial class TrayIcon : Component{
+    sealed partial class TrayIcon : Component{
         public enum Behavior{ // keep order
             Disabled, DisplayOnly, MinimizeToTray, CloseToTray, Combined
         }

Data/CombinedFileStream.cs

@@ -91,7 +91,7 @@ void IDisposable.Dispose(){
             stream.Dispose();
         }
 
-        public class Entry{
+        public sealed class Entry{
             public string Identifier { get; }
 
             public string KeyName{

Data/Serialization/FileSerializer.cs

@@ -95,7 +95,7 @@ public void Read(string file, T obj){
             }
         }
 
-        private class BasicTypeConverter : ITypeConverter{
+        private sealed class BasicTypeConverter : ITypeConverter{
             bool ITypeConverter.TryWriteType(Type type, object value, out string converted){
                 switch(Type.GetTypeCode(type)){
                     case TypeCode.Boolean:

Plugins/Controls/PluginControl.cs

@@ -8,7 +8,7 @@
 using TweetDuck.Plugins.Enums;
 
 namespace TweetDuck.Plugins.Controls{
-    partial class PluginControl : UserControl{
+    sealed partial class PluginControl : UserControl{
         private readonly PluginManager pluginManager;
         private readonly Plugin plugin;
 

Plugins/Events/PluginChangedStateEventArgs.cs

@@ -1,7 +1,7 @@
 using System;
 
 namespace TweetDuck.Plugins.Events{
-    class PluginChangedStateEventArgs : EventArgs{
+    sealed class PluginChangedStateEventArgs : EventArgs{
         public Plugin Plugin { get; }
         public bool IsEnabled { get; }
 

Plugins/Events/PluginErrorEventArgs.cs

@@ -2,7 +2,7 @@
 using System.Collections.Generic;
 
 namespace TweetDuck.Plugins.Events{
-    class PluginErrorEventArgs : EventArgs{
+    sealed class PluginErrorEventArgs : EventArgs{
         public bool HasErrors => Errors.Count > 0;
 
         public IList<string> Errors { get; }

Updates/UpdateHandler.cs

@@ -93,7 +93,7 @@ private void TriggerCheckFinishedEvent(UpdateEventArgs args){
             CheckFinished?.Invoke(this, args);
         }
 
-        public class Bridge{
+        public sealed class Bridge{
             private readonly UpdateHandler owner;
 
             public Bridge(UpdateHandler owner){

subprocess/Program.cs

@@ -20,7 +20,7 @@ private static int Main(string[] args){
             else return SubProcess.ExecuteProcess();
         }
 
-        private class RendererProcess : SubProcess{
+        private sealed class RendererProcess : SubProcess{
             public RendererProcess(string[] args) : base(args){}
 
             public override void OnBrowserCreated(CefBrowserWrapper wrapper){

video/Controls/ControlWMP.cs

@@ -5,7 +5,7 @@
 namespace TweetDuck.Video.Controls{
     [DesignTimeVisible(true)]
     [Clsid("{6bf52a52-394a-11d3-b153-00c04f79faa6}")]
-    class ControlWMP : AxHost{
+    sealed class ControlWMP : AxHost{
         public WindowsMediaPlayer Ocx { get; private set; }
 
         public ControlWMP() : base("6bf52a52-394a-11d3-b153-00c04f79faa6"){}

video/FormPlayer.cs

@@ -8,7 +8,7 @@
 using WMPLib;
 
 namespace TweetDuck.Video{
-    partial class FormPlayer : Form{
+    sealed partial class FormPlayer : Form{
         protected override bool ShowWithoutActivation => true;
 
         private readonly IntPtr ownerHandle;
@@ -258,7 +258,7 @@ private void StopVideo(){
             Close();
         }
 
-        internal class MessageFilter : IMessageFilter{
+        internal sealed class MessageFilter : IMessageFilter{
             private readonly FormPlayer form;
 
             private bool IsCursorOverVideo{