GObject introspection support

Here are a variety of patches adding gobject introspection support to a number of packages along with some version bumps. `pygobject3` has been forked off from the `pygobject` formula, which will be retained to support the 2.0 series. While `pygobject3` has a `with-tests` option to run the package's testsuite, this currently fails on my machine, hanging in `test_tree_view (test_override_gtk.TestTreeView)` with high CPU usage after most other tests have succeeded. Python 3 support would require py3cairo and can be added in the future. GObject Introspection is necessary for Gtk+3 python bindings and thus Gtk+3's dependencies (gdk-pixbuf, atk and pango) require gobject intorospection.