Continuing work on the GTK config box. Created uxcfg.c for the

Continuing work on the GTK config box. Created uxcfg.c for the
Unix-specific config items; moved a stray Windows-specific config
item (scrollbar-in-fullscreen) out into wincfg.c to stop it
appearing on Unix; continued updates to gtkdlg.c. I now believe the
GTK config box looks basically correct (modulo minor cosmetic issues
and keyboard accelerators). Next step, add the event handling so
it's actually functional.

[originally from svn r2933]