Fixes and improves several access key (accelerator) related issues (#17292)

* fix accelerator behavior for menu items and labels

* add elements with matching accelerator to test cycling in sub menus

* Add AccessKeyHandler tests for accelerators with more than one match

* Implement accelerator behavior based on WPF handling

* Remove commented code

* Remove OnAccessKey override => handled by DefaultMenuInteractionHandler

* remove obsolete test

* handle OnAccessKeyPressed for selected tab item

* fix unit tests

* use AccessKeyEvent instead of AccessKeyPressedEvent in unit tests

* navigate menu with and without ALT key

* Revert formatting changes in Tests

* Fix AccessKeyHandler comments

* move private types to bottom

* Remove lock statements, optimize removal of AccessKeyRegistrations

* remove call to Dispatcher.UIThread.Post

* simplifiy AccessKeyHandler.SortByHierarchy

* remove unnecessary method AccessKeyHandler.GetTargetsForSender

* regenerate API suppression file

* revert unneeded changes in MenuPage.axaml

* correct formatting changes

* do not sort by hierarchy if too few targets

* make AccessKeyEventArgs internal

* make AccessKeyPressedEventArgs internal

---------

Co-authored-by: Hans Docsek <hans.docsek@gmail.com>

api/Avalonia.nupkg.xml

@@ -91,4 +91,10 @@
     <Left>baseline/netstandard2.0/Avalonia.Controls.dll</Left>
     <Right>target/netstandard2.0/Avalonia.Controls.dll</Right>
   </Suppression>
+  <Suppression>
+    <DiagnosticId>CP0012</DiagnosticId>
+    <Target>M:Avalonia.Controls.Button.OnAccessKey(Avalonia.Interactivity.RoutedEventArgs)</Target>
+    <Left>baseline/netstandard2.0/Avalonia.Controls.dll</Left>
+    <Right>target/netstandard2.0/Avalonia.Controls.dll</Right>
+  </Suppression>
 </Suppressions>