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.