These events are supposed to carry a pointer to the menu which was opened or
closed, but wxMenuEvent::GetMenu() always returned NULL for the menus opened
when a child MDI frame was active, as its menu bar, containing the menu, was
not searched for it.
Fix this by overriding MSWFindMenuFromHMENU() at wxMDIParentFrame level, just
as we already do for FindItemInMenuBar().
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@78130 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775