style++ && turn backend into a semi-singleton (in that it has a self
member) since there is no phonon-vlc without actual backend instance we
can do that, not terribly nice though as backend only gets accessed to
get hold of the devicemanager. Now since there is also only one DM the
DM should probably be the singleton. For now lets keep it safe tough.
This eliminates the need of holding a reference to the backend in AO and
ADO.