bug#48307: Feature request: provide default keybindings to change the font size in all windows

["Clément Pit-Claudel" <cpitclaudel@gmail.com>]

Hi all,

Emacs has keybindings to change the font size in the current buffer (C-x C-=, etc which use text-scale-adjust).  We don't have keybindings for commands to change the font size globally, and in fact we don't have commands for that purpose at all, AFAICT.

My students keep running into this issue, and it seems pretty common online, too:
- https://stackoverflow.com/questions/18783227/emacs-zoom-in-out-globally
- https://stackoverflow.com/questions/24705984/increase-decrease-font-size-in-an-emacs-frame-not-just-buffer
- https://www.reddit.com/r/emacs/comments/dip6e6/zoom_in_zoom_out/

… and there are many libraries that reimplement solutions to this issue:
- https://www.emacswiki.org/emacs/download/zoom-frm.el
- https://practicalli.github.io/spacemacs/install-spacemacs/change-font.html
- https://github.com/purcell/default-text-scale
- https://www.emacswiki.org/emacs/GlobalTextScaleMode
- https://www.emacswiki.org/emacs/SetFonts#ZoomFrame

I'm sure many more people have custom code for this (I do).  Not all the implementations are good, and it would be very nice if such a feature were available out of the box.

Currently mouse-wheel-text-scale is bound to C-<mouse-5> and C-<mouse-4>; could we have an equivalent command for all buffers, maybe on C-S-<mouse-5> and C-S-<mouse-4>?

Clément.