bug#22000: Patch addressing the menu-bar frame-resize interaction

[martin rudalics]

> I realize that it fixes an annoying misfeature, but it does so by
> introducing a significant new feature which requires quite a few code
> lines.  For a problem that AFAIU has been with us since time
> immemoriam, I'm really uneasy with putting this on emacs-26.  But I'm
> fully prepared to hear arguments to the contrary.

My sole argument is that the feature does not introduce any hidden or
subtle changes.  As long as users have a recent enough GTK installed
and use menu bars any bugs should be easy to find.  The glitch Robert
has detected is a proof for this claim (my GTK was too old for that).

In either case I'll be just happy to put this problem to a rest - on
the release branch or on master.

martin