audio: Refactor message handling to delegate work more.

This should allow for easier testing since we can send the parsed result of the
messages for all message types.
audio: Use on buffering internal in buferring tests
audio: Only trigger buffer pause once.
audio: Expand audio state changed with target state
audio: Add tests for state changes while buffering
core: Ignore buffering for now
Merge remote-tracking branch 'adamcik/fix/gh-734-cleanup-audio-buffering' into develop

Conflicts:
	mopidy/audio/actor.py
docs: Update changelog for PR#763