Fix wxUpdateUIEvent prcoessing for standard menu items under Mac

Handle these events in the menu itself first, then the window and only
then at the application level instead of falling back on the application
before searching the window for the handler.

This requires using the logic already present in the base class
DoProcessEvent() method, so make it protected to allow reuse.

Closes #4769.

include/wx/menu.h

@@ -398,7 +398,7 @@ protected:
     static bool      ms_locked;
 
 
-private:
+protected:
     // Common part of SendEvent() and ProcessMenuEvent(): sends the event to
     // its intended recipients, returns true if it was processed.
     static bool DoProcessEvent(wxMenuBase* menu, wxEvent& event, wxWindow* win);