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/Utils.java

@@ -217,6 +217,9 @@ public class Utils {
 		for (SongSort sort : SongSort.values())
 			sort.init(width, height);
 
+		// initialize hit objects
+		OsuHitObject.init(width, height);
+
 		// back button
 		Image back = GameImage.MENU_BACK.getImage();
 		backButton = new MenuButton(back,
@@ -316,11 +319,11 @@ public class Utils {
 	 */
 	public static void loadCursor() throws SlickException {
 		// destroy old cursors, if they exist
-		if (cursor != null)
+		if (cursor != null && !cursor.isDestroyed())
 			cursor.destroy();
-		if (cursorTrail != null)
+		if (cursorTrail != null && !cursorTrail.isDestroyed())
 			cursorTrail.destroy();
-		if (cursorMiddle != null)
+		if (cursorMiddle != null && !cursorMiddle.isDestroyed())
 			cursorMiddle.destroy();
 		cursor = cursorTrail = cursorMiddle = null;