Properly get track position before change events

In particular, this allows to send the right information when:

1. the track finishes and switches to the next one in the list;
2. user presses next / previous

The cases of EOS and stop event were already handled properly.

Note: we only have GStreamer's `about-to-finish` event to deal with the
end of a track, which usually happens a few seconds before the end of
the track. We set the position to the length of the track, which is not
overridden unless the user generates a relevant callback.

mopidy/core/playback.py

@@ -251,6 +251,10 @@ class PlaybackController(object):
         if self._state == PlaybackState.STOPPED:
             return
 
+        # Unless overridden by other calls (e.g. next / previous / stop) this
+        # will be the last position recorded until the track gets reassigned.
+        self._last_position = self._current_tl_track.track.length
+
         pending = self.core.tracklist.eot_track(self._current_tl_track)
         # avoid endless loop if 'repeat' is 'true' and no track is playable
         # * 2 -> second run to get all playable track in a shuffled playlist
@@ -394,6 +398,10 @@ class PlaybackController(object):
         if not backend:
             return False
 
+        # This must happen before prepare_change gets called, otherwise the
+        # backend flushes the information of the track.
+        self._last_position = self.get_time_position()
+
         # TODO: Wrap backend call in error handling.
         backend.playback.prepare_change()