stream: Start moving state up to backend
stream: Start moving state up to backend

This allows us to start unifying how we handle playlists in the library and
playback cases.

kingosticks/mopidy,ZenithDK/mopidy,vrs01/mopidy,kingosticks/mopidy,jcass77/mopidy,kingosticks/mopidy,mopidy/mopidy,mopidy/mopidy,jodal/mopidy,jcass77/mopidy,vrs01/mopidy,ZenithDK/mopidy,vrs01/mopidy,jodal/mopidy,adamcik/mopidy,ZenithDK/mopidy,adamcik/mopidy,jcass77/mopidy,adamcik/mopidy,jodal/mopidy,mopidy/mopidy,ZenithDK/mopidy,vrs01/mopidy