Set GTK3 as the default, and stop marking it unfinished.

Set GTK3 as the default, and stop marking it unfinished.

Today I've gone through the whole GTK front end, doing a manual test
of every piece of code that I either remembered having had to fiddle
with for GTK3, or suddenly realised I _should_ have fiddled with. I've
fixed all the bugs arising from that exercise; and what with that, the
fact that the new Cairo image surface strategy makes server-side font
handling _faster_ in GTK3 than in GTK2, and the fact that GTK3 also
supports the shiny new smooth scrolling system for touchpads, I
suddenly think that the GTK3 build is now at least as good as GTK2.

So I've switched the configure script over to picking it by default if
it can, and I've also removed the 'unfinished and experimental'
warning if you select it. I for one will now start using GTK3 PuTTY
and pterm for my day-to-day work.