Dim theme song volume when window is not focused.

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

src/itdelatrisu/opsu/audio/MusicController.java

@@ -257,6 +257,14 @@ public class MusicController {
 		return (trackExists() && player.setPosition(position / 1000f));
 	}
 
+	/**
+	 * Sets the music volume.
+	 * @param volume [0, 1]
+	 */
+	public static void setVolume(float volume) {
+		SoundStore.get().setMusicVolume(volume);
+	}
+
 	/**
 	 * Plays the theme song.
 	 */