Playlist: reset song priority on playback

A priority should be a volatile thing: it should schedule a song for
playing back once, but it should not affect the next time the queue
gets played.

src/queue/Playlist.cxx

@@ -64,6 +64,10 @@ void
 playlist::SongStarted()
 {
 	assert(current >= 0);
+
+	/* reset a song's "priority" when playback starts */
+	if (queue.SetPriority(queue.OrderToPosition(current), 0, -1, false))
+		OnModified();
 }
 
 inline void