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