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

[martin rudalics]

> The attached patch series:
>
>   - Lets the user control menubar scrolling via a frame parameter.
>   - Handles initial-frame-alist
>   - Handles default-frame-alist
>   - Handles the modify-frame-parameter path (this is secretly how initial
>     frames work).
>   - Documents the new frame parameter.
>
> Patches prepared on top of the emacs-26 branch but should apply to master
> too.

Thank you very much.  After a quick first evaluation I can confirm
that the menu bar gets truncated and the frame is not resized when
switching to dired with a sufficiently narrow frame.

I'm currently struggling with the following problems:

(1) 0003-GTK3-menubar-resize-scrollbar-behaviour-now-driven-b.patch
doesn't apply here on master.

(2) With all patches applied to the emacs-26 branch I get no initial
menu bar and toggling 'menu-bar-mode' won't get it either.

(3) With patches 1 and 2 applied to master, my menu bar takes more
empty space above and below the text than before.  I had this problem
earlier but I forgot the context.  So this might be related to some
part of my setup here but is certainly triggered by the patch.

I'll get back to you as soon as I find out more about (2) and (3).
Please prepare patches that apply on master so people who use master
only can test them.  And obviously everybody is urged to test this fix
for that longstanding bug.

Thanks again, martin