[Gtk] Upgrade to Gtk+3, GStreamer 1.0 and python-gi

This includes:
* Player (manual) drawing changed to use Cairo
* GenericModel now directly inherits from TreeModel
* lots of syntax changes

Fixes #13.
Fixes #14.

Signed-off-by: Jussi Kukkonen <jussi.kukkonen@intel.com>
[Utils] Make remote image loading asynchronous

Remote images were previously loaded synchronously in
image_from_file(). Change that into a PixbufAsyncLoader that takes a
url and a callback. Callback will be called when pixbuf is ready.

Fixes #12.

Signed-off-by: Jussi Kukkonen <jussi.kukkonen@intel.com>
[Player] set minimum size for images (to avoid bug in GdkPixbuf)

GdkPixbuf.scale_simple() can hang for several seconds if the new
image size is very small: set minimum size to prevent that.

Signed-off-by: Jussi Kukkonen <jussi.kukkonen@intel.com>