player: handle TRACK_CHANGED only on metadata update
player: handle TRACK_CHANGED only on metadata update

The TRACK_CHANGED thing was done on MPV_EVENT_START_FILE, which is dispatched
*before* the actual metadata changes. This means that on track change the
PropertiesChanged signal was sent once with the old track metadata (which is
obviously wrong), and then again with for new track.

ghedo/grooved