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

[Vivek Dasmohapatra]

Hi - have been busy for a while but finally put this together.

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.

Let me know if the series needs any revision (I have already done the 
copyright-assignment dance).

emacs-frame-size-jitter.tar.xz
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