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

interface/wx/event.h

@@ -4107,8 +4107,24 @@ public:
     these do not include menu command events, which are
     handled using wxCommandEvent objects.
 
-    The default handler for @c wxEVT_MENU_HIGHLIGHT displays help
-    text in the first field of the status bar.
+    Events of this class are generated by both menus that are part of a
+    wxMenuBar, attached to wxFrame, and popup menus shown by
+    wxWindow::PopupMenu(). They are sent to the following objects until one of
+    them handles the event:
+
+        -# The menu object itself, as returned by GetMenu(), if any.
+        -# The wxMenuBar to which this menu is attached, if any.
+        -# The window associated with the menu, e.g. the one calling
+        PopupMenu() for the popup menus.
+        -# The top level parent of that window if it's different from the
+        window itself.
+
+    This is similar to command events generated by the menu items, but, unlike
+    them, wxMenuEvent are only sent to the window itself and its top level
+    parent but not any intermediate windows in the hierarchy.
+
+    The default handler for @c wxEVT_MENU_HIGHLIGHT in wxFrame displays help
+    text in the status bar, see wxFrame::SetStatusBarPane().
 
     @beginEventTable{wxMenuEvent}
     @event{EVT_MENU_OPEN(func)}