Implemented dynamic main menu backgrounds.

- This can be switched off in the configuration file.
- Credits: https://osu.ppy.sh/forum/t/98954

Other changes:
- Removed OsuFile setters/getters from Game state, replaced with a 'getOsuFile()' method in MusicController (a more logical location).

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

src/itdelatrisu/opsu/states/SongMenu.java

@@ -602,7 +602,6 @@ public class SongMenu extends BasicGameState {
 		Display.setTitle(String.format("%s - %s", game.getTitle(), osu.toString()));
 		OsuParser.parseHitObjects(osu);
 		SoundController.setSampleSet(osu.sampleSet);
-		Game.setOsuFile(osu);
 		Game.setRestart(Game.RESTART_NEW);
 		game.enterState(Opsu.STATE_GAME, new FadeOutTransition(Color.black), new FadeInTransition(Color.black));
 	}