[CM] gtk in snd?

[bil at ccrma.Stanford.EDU]

Would anyone greatly object if I remove all gtk support
from Snd?  gtk3 was changing so much and so often that
I stopped trying to keep up.  gtk4 is turning out to be
a disaster.  It is a mostly new GUI toolkit based
on crap like XML files, and the drawing area handling
looks unusable to me, though I haven't dived into it
yet.

I will support the Motif version indefinitely in any case.