Enabled application restarts.

- Pressing Ctrl+Shift+F5 in the options menu restarts the game.  Beatmaps and sounds are not reloaded.
- Use GameContainer.setForceExit(false) to trigger a restart after exiting.

Other changes:
- Fixed general issues with track pausing/pause states.
- Store all background images loaded in OsuFiles in a static hash table, instead of in individual objects.  This allows easier access to the allocated memory.
- Only delete OSZ files if they were unzipped. (They were previously deleted in all cases.)
- Moved more images (mods, playfield, lighting) into GameImage.
- Moved OsuHitObject initialization inside Utils.

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

src/itdelatrisu/opsu/audio/MusicController.java

@@ -135,6 +135,7 @@ public class MusicController {
 		if (trackExists()) {
 			setVolume(Options.getMusicVolume() * Options.getMasterVolume());
 			player.setPosition(position / 1000f);
+			pauseTime = 0f;
 			if (loop)
 				player.loop();
 			else
@@ -237,6 +238,8 @@ public class MusicController {
 	public static void stop() {
 		if (isPlaying())
 			player.stop();
+		if (trackExists())
+			pauseTime = 0f;
 	}
 
 	/**
@@ -400,7 +403,7 @@ public class MusicController {
 
 			player = null;
 		} catch (Exception e) {
-			ErrorHandler.error("Failed to destroy OpenAL.", e, false);
+			ErrorHandler.error("Failed to destroy OpenAL.", e, true);
 		}
 	}
 }