Beatmap downloader improvements and fixes.

Updates:
- Added some buttons to downloads menu: clear inactive downloads, import beatmaps, reset search, and show/hide unranked maps.
- Small changes to OsuParser, OszUnpacker, and OsuGroupList (mostly adding return values) to allow parsing only newly unpacked beatmaps.
- Added alpha fade hover effect to MenuButton, as an alternative to expanding (used for 3-part menu buttons).
- Added text rendering fields to MenuButton (also for the 3-part menu buttons).
- Added sound effects to downloads menu.

Fixes:
- Check downloads for illegal filename characters, and remove them if necessary.
- The number of results and downloads shown now supports all resolutions.
- Confirmation dialog no longer appears when restarting the application (since downloads are static).
- Do not set a focus node immediately if the theme song will be played.
- Always play the theme song if no songs are loaded (even if disabled in settings).

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

src/itdelatrisu/opsu/GameImage.java

@@ -441,12 +441,7 @@ public enum GameImage {
 			return img.getScaledCopy(MENU_LOGO.getImage().getWidth() * 0.66f / img.getWidth());
 		}
 	},
-	MENU_BUTTON_MID ("button-middle", "png", false, false) {
-		@Override
-		protected Image process_sub(Image img, int w, int h) {
-			return img.getScaledCopy(w / 2, img.getHeight());
-		}
-	},
+	MENU_BUTTON_MID ("button-middle", "png", false, false),
 	MENU_BUTTON_LEFT ("button-left", "png", false, false),
 	MENU_BUTTON_RIGHT ("button-right", "png", false, false),
 	MUSIC_PLAY ("music-play", "png", false, false),