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

[Gregory Heytings]

> I'm not sure: I don't know if this is the right implementation, or if 
> there's something subtly wrong about it.  It looks good to me (though 
> it's missing a command to go back to the original size), but since there 
> are many implementations that use different strategies I had concluded 
> that there must be some subtlety.

I don't know.  It's certainly possible to do something more complex, but 
this is equivalent to changing the size with Options -> Set Default Font.

> There's something more subtle happening, because using text-scale-adjust 
> is much faster, even on a frame with a single buffer.

Again I don't know, but I don't think so.  AFAICS it's faster because 
redisplaying a single buffer without changing the window and frame size is 
faster than redisplaying all buffers in a frame and resizing the frame 
itself.