Follow-up to #25.

- Moved all MultiClip-handling methods into the MultiClip class.
- Destroy extra clips when starting and finishing new beatmaps.
- Reduced maximum number of extra clips to 20.
- Attempted fixes at multithreading issues.

Signed-off-by: Jeffrey Han <itdelatrisu@gmail.com>

src/itdelatrisu/opsu/states/GameRanking.java

@@ -185,7 +185,6 @@ public class GameRanking extends BasicGameState {
 			retryButton.resetHover();
 			exitButton.resetHover();
 		}
-		SoundController.destroyExtraClips();
 	}
 
 	@Override