Added easing functions for all-around better animations.

These are Robert Penner's easing functions (http://robertpenner.com/easing/), refactored by CharlotteGore to only take a t parameter (https://github.com/CharlotteGore/functional-easing). Licensed under BSD (the former) and MIT (the latter).

Related changes:
- Added "AnimatedValue" utility class for updating values used in animations.
- MenuButton now uses AnimatedValue to handle its animations (still linear by default).
- Added in-out-back easings on logo, mods, and various other elements; added out-bounce easings on button menu.

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

src/itdelatrisu/opsu/states/Game.java

@@ -48,6 +48,7 @@ import itdelatrisu.opsu.replay.Replay;
 import itdelatrisu.opsu.replay.ReplayFrame;
 import itdelatrisu.opsu.ui.MenuButton;
 import itdelatrisu.opsu.ui.UI;
+import itdelatrisu.opsu.ui.animations.AnimationEquation;
 
 import java.io.File;
 import java.util.LinkedList;
@@ -1425,6 +1426,8 @@ public class Game extends BasicGameState {
 			Image skip = GameImage.SKIP.getImage();
 			skipButton = new MenuButton(skip, width - skip.getWidth() / 2f, height - (skip.getHeight() / 2f));
 		}
+		skipButton.setHoverAnimationDuration(350);
+		skipButton.setHoverAnimationEquation(AnimationEquation.IN_OUT_BACK);
 		skipButton.setHoverExpand(1.1f, MenuButton.Expand.UP_LEFT);
 
 		// load other images...