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
Merge branch 'develop' into feature/audio-prep-work-for-gapless
flake8: Fix duplicate import and import order in audio actor test
Merge branch 'develop' into feature/audio-prep-work-for-gapless
review-fixes: Mostly typos etc.
Merge pull request #680 from adamcik/feature/audio-prep-work-for-gapless

audio: Preparation work for gapless playback support