Detangle EOT and EOS.

This commit tries to detangle EOS from EOT which we have incorrectly
intermingled. EOS events should only be happening at the end of the playlist
when we are about to stop. EOT handling has been removed / broken in this
change, and will need to be re-added with proper tests.
Add change track to playback providers.

This method is intended to make implementing EOT handling much more streamlined
by adding a way to easily just change the URI and any other state without
messing with gstreamer pipe states

Naming of this is a bit unfortunate as it is not the same as the
core.playback's concept of change_track. At least not yet
Use source's factory name to determine type.

Checking the URI inside the new-source handler turned out to tickle some issue
that gave us deadlocks. Getting the source and checking which factory created
it is much safer.
Setup about to finish handling for proper EOT support.

With this change we ask the core.playback system to set the next tracks uri and
otherwise update state as needed. With this change we should be able to support
streaming oggs and gapless playback to mention a few of items this fixes.

On the down side, this change breaks test_end_of_track_skips_to_next_track_on_failure
as there is no clean way to handle this with the async nature of this EOT
handling.
audio: Remove blocking get_state() calls in get_position() and seek()
audio: Setup appsrc with seek-data callback
spotify: Replace wall clock timer with GStreamer timer
audio: Log buffering messages
audio: Add appsrc need-data and enough-data callbacks
audio: Make appsrc buffer between 0.5 and 1 MB of data
spotify: Only push audio data when GStreamer wants more
audio: Move Spotify appsrc caps out of audio
Merge branch 'feature/spotify-to-use-gst-time' into feature/limit-spotify-data-pushing

Conflicts:
	mopidy/audio/actor.py
	mopidy/backends/spotify/playback.py
docs: Add v0.12 section to changelog
Merge pull request #289 from jodal/feature/spotify-to-use-gst-time

Make Spotify backend use GStreamer for time position and seek
docs: Update changelog
Merge branch 'develop' into feature/limit-spotify-data-pushing
audio: Add MB constant to make code more readable
spotify: Hook need-data and enough-data callbacks onto appsrc
spotify: Reduce callback logging