Added GTK support to ZeroMQ kernel. We use an approach which is a combination of an gtk timer callback into our execution loop, like we do for Qt and Wx, I've run as tests several GTK examples found on the net, as well as multiple matplotlib scripts, and so far everything works as expected. The only catch is that we silently trap gtk.main_quit(), so examples that call it with a 'close' button or similar seem to not do anything. But their windows close normally and no other problems have been found. This solution uses code taken from an old bug report of ours: https://bugs.launchpad.net/ipython/+bug/270856 specifically the attachment in this comment: https://bugs.launchpad.net/ipython/+bug/270856/comments/6 along with the changes suggested by Michiel de Hoon there. Thanks to Ville and Michiel for that old discussion, which put me on the right track to figure out the details of the logic needed for GTK.