1. GetMenuBar() tries GetParent()->GetMenuBar() first (traverse up hierarchy).
2. IsAttached() returns true if GetMenuBar() returns a menubar.
3. AddSubMenu() does not call Attach() on added submenus.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@26477 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775