Merge branch 'feature/metadata-provider' into pu

Conflicts:
	mopidy/backends/base.py
	mopidy/core/actor.py
Merge branch 'feature/mpd-playlist-management' into pu
Merge branch 'feature/playback-error-notify' into pu
Merge branch 'feature/split-audio-and-volume-config' into pu
Merge branch 'fix/mpd-output-status' into pu
Merge branch 'feature/playback-error-notify' into pu