Gapless: fix smooth transition for RepeatSingle mode (bgo#627391)

RepeatSingle mode was setting the next track with OpenPlay() instead
of using SetNextTrack() when receiving the RequestNextTrack event,
which meant that the track to play next was not received in time*,
and therefore not succeed in making the transition gapless.

* The consequence of this was the following debug message:
[23 Debug 21:47:47.275] [Gapless] Timed out while waiting for next_track_set to be raised