Don't depend on gnome 2 python libs
- Remove old gnome 2 python libs from BuildRequires (#716719)
- Depend on libwnck3 instead of gnome-python2-libwnck, as Orca now uses
Wnck through g-i
- Require pyxdg (#761306)
This will get rid of libwnck + gnome-python2-libwnck on the Desktop Live
CD.