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/SongMenu.java

@@ -209,7 +209,6 @@ public class SongMenu extends BasicGameState {
 
 		// song button background & graphics context
 		Image menuBackground = GameImage.MENU_BUTTON_BG.getImage();
-		OsuGroupNode.setBackground(menuBackground);
 
 		// song button coordinates
 		buttonX = width * 0.6f;
@@ -651,6 +650,13 @@ public class SongMenu extends BasicGameState {
 		if (MusicController.isThemePlaying() && focusNode != null)
 			MusicController.play(focusNode.osuFiles.get(focusNode.osuFileIndex), true);
 
+		// reset music track
+		else if (resetTrack) {
+			MusicController.pause();
+			MusicController.playAt(MusicController.getOsuFile().previewTime, true);
+			resetTrack = false;
+		}
+
 		// unpause track
 		else if (MusicController.isPaused())
 			MusicController.resume();
@@ -664,13 +670,6 @@ public class SongMenu extends BasicGameState {
 			GameImage.destroySkinImages();  // destroy skin images, if any
 			resetGame = false;
 		}
-
-		// reset music track
-		if (resetTrack) {
-			MusicController.pause();
-			MusicController.playAt(MusicController.getOsuFile().previewTime, true);
-			resetTrack = false;
-		}
 	}
 
 	@Override