Can we go GTK-only?

[Daniel Colascione <dancol@dancol.org>]

I've been doing some work on xterm.c and friends lately --- it's a very 
complex subsystem made even more complex by supporting five different 
toolkits in the same piece of code. GTK support adds a special twist: 
GTK is nothing like an old-fashioned Xt-style toolkit, and trying to 
force it into the mold requires going behind GTK's back in ways that are 
fragile, inefficient, and unsupported. New versions of GTK regularly 
break Emacs. Emacs cannot take advantage of GTK's support for non-X 
targets, like Wayland.

Do we really need all the legacy infrastructure in xterm? Do many people 
really build Emacs as a Motif application? The X window subsystem would 
be made much simpler if we made it GTK3-only.

If we did, much of the complexity of the existing code would evaporate, 
since we'd be using GTK's infrastructure instead of our own. We'd turn 
Emacs frames into well-behaved GTK widgets --- just run redisplay inside 
the Emacs-widget's draw method, draw onto the Cairo surface GTK 
provides. Instead of flushing frames and drawing to the screen directly 
from input-waiting routines, we'd rely exclusively on dirtying the Emacs 
widget and letting the GTK's normal Expose-event processing take care of 
invoking redisplay.

We'd have to get rid of the SIGIO input subsystem too, but I think 
that's a good thing.

The size of the xterm codebase would shrink by at least two thirds, I 
think. We already _have_ Cairo support: sure, it's broken, but the part 
that's broken is integrating with the horrible Xt hacks, not the drawing 
itself.

As a stepping stone, we should at least remove support for the 
no-toolkit, Motif, and Athena configurations. (Lucid I can understand.)