Gtk 3, GStreamer 1.0

This branch includes 
* gtk and gstreamer update
* PixbufAsyncLoader, implemented as well as seems possible using gi.Gio API
* workaround for a GdkPixbuf bug

Feel free to only pick the first one if that seems better.

The new dependencies would be:
* GTK+ 3.0 (I believe the python-gi makes this >=3.4 in reality. Tested with 3.4.2)
* GStreamer 1.0 (tested with 1.0.5)
* python-gi 3.4 (tested with 3.4.2)

The only legacy component left would be dbus, and that seems to still work ok for us: Moving to GDBus might make sense but does not need to be done in the same commit.