Implemented skinnable theme songs.

Plays a theme song when opsu! starts (can be disabled in options).  The default song is "welcome to osu!" by nekodex, uploaded by CyberKitsune.

To change the song, place a new "theme.osu" in the skins folder and edit the file name, metadata, and song length (at minimum).

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

src/itdelatrisu/opsu/states/SongMenu.java

@@ -501,6 +501,10 @@ public class SongMenu extends BasicGameState {
 	public void enter(GameContainer container, StateBasedGame game)
 			throws SlickException {
 		Display.setTitle(game.getTitle());
+
+		// stop playing the theme song
+		if (MusicController.isThemePlaying() && focusNode != null)
+			MusicController.play(focusNode.osuFiles.get(focusNode.osuFileIndex), true);
 	}
 
 	@Override