Mutable and subclassable version of ObservableGroup (#3526)

## Fixes #3519
<!-- Add the relevant issue number after the "#" mentioned above (for ex: Fixes #1234) which will automatically close the issue once the PR is merged. -->

<!-- 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?
<!-- Please uncomment one or more that apply to this PR. -->

<!-- - 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: MicrosoftDocs/WindowsCommunityToolkitDocs#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

<!-- If this PR contains a breaking change, please describe the impact and migration path for existing applications below. 
     Please note that breaking changes are likely to be rejected. -->


## Other information

Microsoft.Toolkit/Collections/ObservableGroup.cs

@@ -4,6 +4,7 @@
 
 using System.Collections.Generic;
 using System.Collections.ObjectModel;
+using System.ComponentModel;
 using System.Diagnostics;
 using System.Linq;
 
@@ -16,8 +17,13 @@ namespace Microsoft.Toolkit.Collections
     /// <typeparam name="TKey">The type of the group key.</typeparam>
     /// <typeparam name="TValue">The type of the items in the collection.</typeparam>
     [DebuggerDisplay("Key = {Key}, Count = {Count}")]
-    public sealed class ObservableGroup<TKey, TValue> : ObservableCollection<TValue>, IGrouping<TKey, TValue>, IReadOnlyObservableGroup
+    public class ObservableGroup<TKey, TValue> : ObservableCollection<TValue>, IGrouping<TKey, TValue>, IReadOnlyObservableGroup
     {
+        /// <summary>
+        /// The cached <see cref="PropertyChangedEventArgs"/> for <see cref="Key"/>
+        /// </summary>
+        private static readonly PropertyChangedEventArgs KeyChangedEventArgs = new PropertyChangedEventArgs(nameof(Key));
+
         /// <summary>
         /// Initializes a new instance of the <see cref="ObservableGroup{TKey, TValue}"/> class.
         /// </summary>
@@ -48,10 +54,24 @@ public ObservableGroup(TKey key, IEnumerable<TValue> collection)
             Key = key;
         }
 
+        private TKey key;
+
         /// <summary>
-        /// Gets the key of the group.
+        /// Gets or sets the key of the group.
         /// </summary>
-        public TKey Key { get; }
+        public TKey Key
+        {
+            get => this.key;
+            set
+            {
+                if (!EqualityComparer<TKey>.Default.Equals(this.key, value))
+                {
+                    this.key = value;
+
+                    OnPropertyChanged(KeyChangedEventArgs);
+                }
+            }
+        }
 
         /// <inheritdoc/>
         object IReadOnlyObservableGroup.Key => Key;