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/states/Splash.java

@@ -119,16 +119,19 @@ public class Splash extends BasicGameState {
 			thread = new Thread() {
 				@Override
 				public void run() {
-					File beatmapDir = Options.getBeatmapDir();
+					// application restart: everything already loaded
+					if (OsuGroupList.get().size() < 1) {
+						File beatmapDir = Options.getBeatmapDir();
 
-					// unpack all OSZ archives
-					OszUnpacker.unpackAllFiles(Options.getOSZDir(), beatmapDir);
+						// unpack all OSZ archives
+						OszUnpacker.unpackAllFiles(Options.getOSZDir(), beatmapDir);
+	
+						// parse song directory
+						OsuParser.parseAllFiles(beatmapDir, width, height);
 
-					// parse song directory
-					OsuParser.parseAllFiles(beatmapDir, width, height);
-
-					// load sounds
-					SoundController.init();
+						// load sounds
+						SoundController.init();
+					}
 
 					finished = true;
 				}