We should use *one* toolkit, and use that as a sane base.
I must say SDL could be used (even though it's updated every 10 years or so), or something else (ClanLib? haven't tried it).
But I would *definitely* not use GTK, since it's a real pain...
Our requirements should be to have a cross-platform toolkit, though, of course

So maybe making a small abstraction layer, and working primarily with a SDL output, but letting people substitute for something else?