GTK3 backend

[GoogleCodeExporter]

To use Python 3 with GTK, we'll need to use GTK3/pyGI, instead of the current
GTK2/pyGTK. Therefore, we need to work on a GTK3 backend.

Before we can do this, though, we'll a GTK3 version of the gtkglext widget.
This is in progress (see
http://www.mail-archive.com/[email protected]/index.html) but not yet
widely distributed. So this is really just a reminder to work on this once the
architecture is in place.

Original issue reported on code.google.com by [email protected] on 11 Sep 2012 at 6:28