## Fixes#3519
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<!-- Add a brief overview here of the feature/bug & fix. -->
Removes two restrictions from `ObservableGroup` to make it usable in more situations. In my case, I wanted to be able to rename the groups (for this I needed a mutable key), and I wanted to add properties on the group level (I needed to unseal it)
- Removes the immutable restriction on the `Key` property and makes the `Key` property observable.
- Removes the `sealed` keyword from `ObservableGroup`
## PR Type
What kind of change does this PR introduce?
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<!-- - Bugfix -->
- Feature
<!-- - Code style update (formatting) -->
<!-- - Refactoring (no functional changes, no api changes) -->
<!-- - Build or CI related changes -->
<!-- - Documentation content changes -->
<!-- - Sample app changes -->
<!-- - Other... Please describe: -->
## What is the current behavior?
<!-- Please describe the current behavior that you are modifying, or link to a relevant issue. -->
`ObservableGroup` has an immutable `Key` property
`ObservableGroup` is not inheritable
## What is the new behavior?
<!-- Describe how was this issue resolved or changed? -->
`ObservableGroup` has a mutable `Key` property
`ObservableGroup` is inheritable
## PR Checklist
Please check if your PR fulfills the following requirements:
- [ ] Tested code with current [supported SDKs](../readme.md#supported)
- [x] Pull Request has been submitted to the documentation repository [instructions](..\contributing.md#docs). Link: https://github.com/MicrosoftDocs/WindowsCommunityToolkitDocs/pull/394
- [x] Sample in sample app has been added / updated (for bug fixes / features) --> _no update needed_
- [x] Icon has been created (if new sample) following the [Thumbnail Style Guide and templates](https://github.com/windows-toolkit/WindowsCommunityToolkit-design-assets) --> _not applicable_
- [x] Tests for the changes have been added (for bug fixes / features) (if applicable) --> _not applicable_
- [x] Header has been added to all new source files (run *build/UpdateHeaders.bat*) --> _not applicable_
- [ ] Contains **NO** breaking changes
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## Other information
