queue/PlaylistControl: don't skip highest priority song on "play"

When starting playback with a specific song which does not have the
highest priority, the previous highest priority song was skipped
completely because its order was "swapped".  This commit changes to a
more expensive operation which inserts the selected song into the
order list.

This fixes a small part of #165

src/queue/PlaylistControl.cxx

@@ -75,8 +75,7 @@ playlist::MoveOrderToCurrent(unsigned old_order)
 	} else {
 		/* not playing anything: move the specified song to
 		   the front */
-		queue.SwapOrders(old_order, 0);
-		return 0;
+		return queue.MoveOrderBefore(old_order, 0);
 	}
 }