Seal PluginConfig and add a method to disable official plugins from config

Plugins/PluginConfig.cs

@@ -4,7 +4,7 @@
 
 namespace TweetDck.Plugins{
     [Serializable]
-    class PluginConfig{
+    sealed class PluginConfig{
         [field:NonSerialized]
         public event EventHandler<PluginChangedStateEventArgs> PluginChangedState;
 
@@ -33,5 +33,9 @@ public void SetEnabled(Plugin plugin, bool enabled){
         public bool IsEnabled(Plugin plugin){
             return !Disabled.Contains(plugin.Identifier) && plugin.CanRun;
         }
+
+        public void DisableOfficialFromConfig(string pluginName){
+            Disabled.Add("official/"+pluginName);
+        }
     }
 }