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/states/GamePauseMenu.java

@@ -196,9 +196,9 @@ public class GamePauseMenu extends BasicGameState {
 			SoundController.playSound(SoundEffect.FAIL);
 		} else
 			MusicController.pause();
-		continueButton.setScale(1f);
-		retryButton.setScale(1f);
-		backButton.setScale(1f);
+		continueButton.resetHover();
+		retryButton.resetHover();
+		backButton.resetHover();
 	}
 
 	/**
@@ -212,5 +212,8 @@ public class GamePauseMenu extends BasicGameState {
 		continueButton = new MenuButton(GameImage.PAUSE_CONTINUE.getImage(), width / 2f, height * 0.25f);
 		retryButton = new MenuButton(GameImage.PAUSE_RETRY.getImage(), width / 2f, height * 0.5f);
 		backButton = new MenuButton(GameImage.PAUSE_BACK.getImage(), width / 2f, height * 0.75f);
+		continueButton.setHoverExpand();
+		retryButton.setHoverExpand();
+		backButton.setHoverExpand();
 	}
 }