Consistently set wxMenuBar parent in all ports.

Consistently set wxMenuBar parent in all ports.

Previously, wxGTK used the associated frame as wxMenuBar parent, while the
other ports did not. Do set the frame as menu bar parent in all ports now.

Of course, considering that wxMenuBar is not a window at all, this doesn't
necessarily make much sense, but at least it makes the behaviour consistent.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73923 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775