mixer: Add mixer API
docs: Add Mopidy-ALSAMixer to mixer impl list
mixer: Add name attr to mixer API
main: Start/stop the selected mixer
mixer: Add bundled Mopidy-SoftwareMixer extension
core: Use new mixer API
mixer: Use initial volume from audio/mixer_volume
audio: Only expose GStreamer's software mixer
audio: Remove custom GStreamer mixers
config: Deprecate audio/mixer_track
docs: Remove MIXER_TRACK_* attribute workaround
docs: Update changelog with mixer changes
docs: Add Mopidy-SoftwareMixer to the mixer impl list
docs: Update audio/* config docs
mixer: Remove 'should' from docstring
main: Stop only the mixer actor that we now is running
swmixer: Remove unused attribute
exc: Test ScannerError
exc: Add {Backend,Frontend,Mixer}Error exceptions
mixer: Mark methods that may be implemented by subclasses