bug#51490: Show an indicator when Emacs is busy somewhere in the Emacs window

[Gregory Heytings]

> > Aren't they the simplest graphical element drawn by Emacs, a simple 
> > black-and-white bitmap?
>
> No, they aren't.  They are certainly not black-and-white: they use 
> faces.

Sorry, I wrote a bit too fast (as usual), I did not mean literally "black" 
and "white", I meant "two colors", represented by a two-value pixel 
bitmap.

> Drawn how?  Bitmaps are images, and drawing images in the text area 
> means we need to invoke all the machinery of redrawing a certain screen 
> line, be it on the mode line or elsewhere.  That's how Emacs draws 
> stuff; we don't have any way of directly poking the glass with an 
> arbitrary bunch of pixels, at least not one that I know of.

I'm thinking aloud here, sorry if it isn't useful.  But I think we have at 
least one similar occurrence: XTflash.