[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>
[UPnP] Do not make duplicate found_server_callback() calls

Use the same code for GetServers() callback and the FoundServer
handler: GetServers() code path wasn't checking for existing servers,
so duplicates could appear with good timing.

Also mark signal handlers as 'private' methods.

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