FluentAvalonia v2-preview4

[amwx]

Updates to Avalonia11.0-preview4

Changes that had to be made to support preview4, since it was apparently decided pointer and key eventargs shouldn't be publicly construct-able:

• MenuFlyoutItemBase & MenuFlyoutSubItem now reuse the existing PointerEntered and PointerExited event args and just change the RoutedEvent to the MenuItem.PointerEntered[Exited]ItemEvent. No idea if this has side-effects, but seems to work
• TextCommandBarFlyout Undo/Redo now uses the RawInputEvent (lower-level) to raise the necessary keystrokes for triggering Undo or Redo. I've also updated this to iterate over the key configurations in the PlatformHotKeyConfiguration rather than hard-coding to Ctrl-Z & Ctrl-Y. No other TextCommandBarFlyout behaviors have been changed or implemented to support newer features (will go on the list though)

Please remember that preview versions may have breaking changes between versions causing incompatibility between this library and upstream with version mismatches. Either wait until I update the package here or tread carefully if using a new upstream preview.