Update reactive tree item

[ChrisPulman]

Update view bindings, tree controls, and package versions.

• MAUI: raised Android SupportedOSPlatformVersion from 21.0 to 23.0.
• Gallery views: bind PersonView and PetView to DisplayName instead of Name.
• ReactiveTreeItem: batch-add children via Children.Edit, set child parent, and add Dispose override to dispose Children.
• ReactiveTreeView: use ReactiveUI.SourceGenerators [IViewFor] attribute, register view with Splat generically, replace direct ViewModel DP plumbing with reactive WhenAnyValue/Subscribe to Children.CurrentItems.
• ReactiveTreeViewModel: add Dispose override to dispose Children.
• TreeView: implement GetContainerForItemOverride, IsItemItsOwnContainerOverride, and robust OnSelectedItemChanged to update SelectedItemProperty.
• CrissCross.WPF.UI: replace external ReactiveList package with a local ProjectReference and add ReactiveList project to solution.
• Directory.Packages.props: bump several package versions (Avalonia, ReactiveUI, ReactiveUI.Avalonia, Microsoft.Extensions packages, Microsoft.Xaml.Behaviors.Wpf, Polyfill, Microsoft.SourceLink.GitHub).

These changes fix bindings, improve tree view selection/container behavior, ensure children lists are disposed to avoid leaks, and update dependencies to newer versions.

Add first-class icon support for reactive tree items and their templates.

• Person: initialize sample Item icon (ImageIcon) and add BitmapImage using directive.
• ReactiveTreeItem: add reactive Icon backing field.
• ReactiveTreeView.xaml: provide ItemContainerStyle to bind IsSelected/IsExpanded and Icon to the item container.
• TreeView.xaml: add DefaultReactiveTreeViewStyle and register it so ReactiveTreeView gets a default template supporting scrolling and virtualization.
• TreeViewItem: add IconVisibility dependency property, update Icon setter to toggle visibility, add IconPropertyChanged handler, and override container methods so items are TreeViewItem containers.
• TreeViewItem.xaml: bind icon Visibility to IconVisibility, adjust layout/min widths and focus visual resource.
• ControlMixins: remove unused using directives.

These changes allow icons to be provided by view models and displayed/hidden automatically in tree item templates, with a default ReactiveTreeView style and sample usage in the gallery.

Replace custom disposable/subject-based tree traversal with pure Rx operators. The patch removes unused usings, adds null checks (NET8_0_OR_GREATER conditional), and rewrites FlattenAndSelect/Flatten to use Select, Switch, Concat and Merge. Introduces helper methods FlattenItems and FlattenItem to produce IObservable streams from root lists and children, simplifying subscription management and eliminating manual CompositeDisposable/ReplaySubject logic. Also updates XML comments to better describe parameters and return values.