core: Enable gapless playback

There is still quite a bit to be done is this area:

- We need to start tracking pending tracks as there is time window when we are
  decoding a new track but still playing the old one
  - Currently this breaks seek, next, prev during this time window
- The old on_end_of_track code needs to die.
- Stream changes need to trigger playing events and switch the pending track to
  current.

mopidy/core/playback.py

@@ -25,12 +25,32 @@ class PlaybackController(object):
         self._volume = None
         self._mute = False
 
+        if self._audio:
+            self._audio.set_about_to_finish_callback(self._on_about_to_finish)
+
     def _get_backend(self, tl_track):
         if tl_track is None:
             return None
         uri_scheme = urlparse.urlparse(tl_track.track.uri).scheme
         return self.backends.with_playback.get(uri_scheme, None)
 
+    def _on_about_to_finish(self):
+        original_tl_track = self.current_tl_track
+
+        next_tl_track = self.core.tracklist.eot_track(self.current_tl_track)
+        # TODO: this should be self.pending_tl_track and stream changed should
+        # make it current.
+        self.current_tl_track = next_tl_track
+
+        backend = self._get_backend(next_tl_track)
+
+        if backend:
+            backend.playback.change_track(next_tl_track.track).get()
+            # TODO: this _really_ needs to be stream changed...
+            self._trigger_track_playback_started()
+
+        self.core.tracklist.mark_played(original_tl_track)
+
     # Properties
 
     def get_current_tl_track(self):
@@ -326,6 +346,7 @@ class PlaybackController(object):
         :type time_position: int
         :rtype: :class:`True` if successful, else :class:`False`
         """
+        # TODO: seek needs to take pending tracks into account :(
         if not self.core.tracklist.tracks:
             return False