Merge pull request #97 from x2b/gtk3

Port to gtk3
Updated requirements and fixed gtk.h bug.

Eventually I think we'll want to downgrade requirements for gdk-3 to
gdk-2; I think we just need to fix the new cursor from surface commands
in gui/Cursor.cpp. Finding where to put g_init_type() to enable glib
2.32 compilations would also be helpful.