GtkSharp/sample/GtkDemo

This is a mostly complete port of gtk-demo to Gtk#; for notes on
what's missing, see the TODO file.

The original port was done by Daniel Kornhauser (dkor@alum.mit.edu),
with additions/changes/fixes by various other people, as seen in the
main gtk-sharp ChangeLog.

For the most part, the various demos should stay as close as possible
to the C version, so that GtkDemo.exe can be compared against gtk-demo
to make sure Gtk# is working correctly.