Harmonize wxMenuEvent handling between all major ports.

Send these events to the menu itself first, then to the menu bar containing
it or the window invoking it if it's a popup menu and, finally, to the top
level window in all of wxGTK, wxMSW and wxOSX.

In particular, this ensures that help strings are now shown in the parent MDI
frame status bar by default, even when the menus are attached to the client
MDI frame or shown as popup menus.

At the implementation level, this logic is now encapsulated in a new static
wxMenu::ProcessMenuEvent() method which can be easily modified and reused in
other ports.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@78230 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

docs/changes.txt

@@ -93,6 +93,7 @@ All (GUI):
 - Add wxUIActionSimulator::Select().
 - Add wxOwnerDrawnComboBox::Is{List,Text}Empty() methods.
 - Fix creating/removing mode buttons in wxPG manager (Artur Wieczorek).
+- Harmonize wxMenuEvent handling between all major ports.
 
 wxGTK: