audio: Add debug logging of stream switching
audio: Queue audio sink data to give us some headroom for about to finish events
core: Add a bunch of TODOs
audio: Add stream_changed event plus tests
audio: Slight test refactor to not hide what is going on
audio: Add more tests for audio events
audio: Add position_changed event to know when seeks happen
audio: Add about to finish callback support
audio: Debug log position_changed event triggering
audio: Update dummy audio used for testing
audio: Update tests and dummy audio actor to pass
audio: Add more TODOs
Merge branch 'develop' into feature/audio-prep-work-for-gapless

Conflict files fixed up to a state where tests pass again before commit.

Conflicts:
	mopidy/audio/actor.py
	mopidy/audio/listener.py
	tests/audio/test_actor.py
	tests/audio/test_listener.py
Workaround for RasPi selecting jack server.
Fixed long line Flake8 warning.
Checks for musicbrainz album ID and sets images to a list containing the corresponding URL
Correct if .. is not None
Test with multiple versions of tornado. See #798
Merge pull request #806 from dz0ny/tornado_matrix

Test with multiple versions of tornado.
Merge pull request #802 from ifosch/697

audio: Scanner adds images with the URL from musicbrainz album ID