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/ui/animations/AnimatedValue.java

@@ -0,0 +1,124 @@
+/*
+ * opsu! - an open-source osu! client
+ * Copyright (C) 2014, 2015 Jeffrey Han
+ *
+ * opsu! is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * opsu! is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with opsu!.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+package itdelatrisu.opsu.ui.animations;
+
+import itdelatrisu.opsu.Utils;
+
+/**
+ * Utility class for updating a value using an animation equation.
+ */
+public class AnimatedValue {
+	/** The animation duration, in milliseconds. */
+	private int duration;
+
+	/** The current time, in milliseconds. */
+	private int time;
+
+	/** The base value. */
+	private float base;
+
+	/** The maximum difference from the base value. */
+	private float diff;
+
+	/** The current value. */
+	private float value;
+
+	/** The animation equation to use. */
+	private AnimationEquation eqn;
+
+	/**
+	 * Constructor.
+	 * @param duration the total animation duration, in milliseconds
+	 * @param min the minimum value
+	 * @param max the maximum value
+	 * @param eqn the animation equation to use
+	 */
+	public AnimatedValue(int duration, float min, float max, AnimationEquation eqn) {
+		this.time = 0;
+		this.duration = duration;
+		this.value = min;
+		this.base = min;
+		this.diff = max - min;
+		this.eqn = eqn;
+	}
+
+	/**
+	 * Returns the current value.
+	 */
+	public float getValue() { return value; }
+
+	/**
+	 * Returns the current animation time, in milliseconds.
+	 */
+	public int getTime() { return time; }
+
+	/**
+	 * Sets the animation time manually.
+	 * @param time the new time, in milliseconds
+	 */
+	public void setTime(int time) {
+		this.time = Utils.clamp(time, 0, duration);
+		updateValue();
+	}
+
+	/**
+	 * Sets the animation duration.
+	 * @param duration the new duration, in milliseconds
+	 */
+	public void setDuration(int duration) {
+		this.duration = duration;
+		int newTime = Utils.clamp(time, 0, duration);
+		if (time != newTime) {
+			this.time = newTime;
+			updateValue();
+		}
+	}
+
+	/**
+	 * Sets the animation equation to use.
+	 * @param eqn the new equation
+	 */
+	public void setEquation(AnimationEquation eqn) {
+		this.eqn = eqn;
+		updateValue();
+	}
+
+	/**
+	 * Updates the animation by a delta interval.
+	 * @param delta the delta interval since the last call.
+	 * @return true if an update was applied, false if the animation was not updated
+	 */
+	public boolean update(int delta) {
+		int newTime = Utils.getBoundedValue(time, delta, 0, duration);
+		if (time != newTime) {
+			this.time = newTime;
+			updateValue();
+			return true;
+		}
+		return false;
+	}
+
+	/**
+	 * Recalculates the value by applying the animation equation with the current time.
+	 */
+	private void updateValue() {
+		float t = eqn.calc((float) time / duration);
+		this.value = base + (t * diff);
+	}
+}