audio: Split out the signals tracking
audio: Add docs to signal handler helper
audio: Split out appsrc handling
audio: Use pbutils to provide usable plugin missing info
audio: Improve GStreamer logging

This adds an extra mopidy.audio.gst logger and moves the GStreamer logging to
it. Additionally this adds more logging so we can likely get by with just
mopidy logs in more cases.
audio: Split out ouput handling from audio actor

This also lays some basic ground work for handling multiple outputs.
audio: Unify source handlers

notify::source and setup-source are the same, just that setup-source is a
convenience wrapper.
audio: Split out proxy setup
audio: Refactor softwaremixer and audio interactions

This rips the mixer bits and pieces that have been hiding in the audio actor to
it's own class. The software mixer now only knows about this and nothing else
from audio.
audio: Group playbin teardown/setup

This makes it possible to see that we setup and teardown the same things. Also
fixes disconnect on a signal we no longer listen to.
audio: Annotate code with more TODOs and questions
Merge pull request #814 from adamcik/feature/audio-actor-refactor

mopidy.audio.actor refactoring and cleanups