Add Tweening framework.

[nightm4re94]

This PR adds a package for interpolating values over time, offering predefined Easing functions defined by Robert Penner as well as the possibility to define custom mathematical tweening equations. If an object implements the Tweenable interface, it can override the getTweenValues and setTweenValues methods to define which attributes will be modified.
Current examples for Tweenables are Entities(position, dimensions, angle, hitpoints, collision dimensions, velocity) and GuiComponents (position, dimensions, text angle), as well as SoundPlaybacks (volume).
Each game has one instance of a TweenEngine which operates as a manager for all registered Tween instances and applies their TweenEquation with each update. It can be retrieved using Game.tweens().
Tweens can be stopped, resumed, and reset, as the examples below show:

GuiComponent comp = new GuiComponent();

// Start a Tween or restart it, if there is already a Tween. In this case, the width and height of the GuiComponent will be decreased by 10 over the course of 500 milliseconds, using an elastic easing function.
Game.tweens().begin(comp, TweenType.SIZE_BOTH, 500).targetRelative(-10, -10).ease(TweenFunction.ELASTIC_OUT);

// Stop a Tween
Game.tweens().stop(comp, TweenType.SIZE_BOTH);

// Resume a Tween
Game.tweens().resume(comp, TweenType.SIZE_BOTH);

// Reset the component's width and height to how they were before tweening
Game.tweens().reset(comp, TweenType.SIZE_BOTH);

Apart from the Tweening framework, this PR implements the following changes:

• Add a listener to receive events when a key is released
• Add setters for row height and column with in ImageComponentLists
• Remove distinction between Particle color alpha and opacity. (Fix flickering particle opacity when fading.)

[steffen-wilke]

• Document changes on Particle and IKeyboard.
• Adjust licensing and remove "unneccessary" easing functions.

TODO:

• Loopable Tweens
• Missing events of tweens
• Possibility to determine whether a TweenType is supported by a Tweenable

[nightm4re94]

Last change adressed these remarks:

• Possibility to determine whether a TweenType is supported by a Tweenable
• Document changes on Particle and IKeyboard.
• Adjust licensing and remove "unneccessary" easing functions.

[TheRamenChef]

It seems I came just a few hours too late to offer my feedback on this PR.

First of all, the complete list of VolumeControl instances should not be exposed publicly, nor should it be accessed anywhere other than the updateVolume method, because a method iterating over it does not know what any of them are supposed to represent. And the list is grossly misused by the various fade methods, which currently fade out every volume control attached to the playback instance. They should either create an independent volume control for it or use the master volume control (as the fade method seems to be mainly concerning the master volume).

On the subject of the fade method also, the original fadeMusic method was supposed to make the music fade out, and would stop the playback as soon as the volume reached zero. It no longer does this in the current version.

Second, it seems that TweenType and Tweenable could be replaced with interfaces to get and set tweened values (or just use DoubleSupplier and DoubleConsumer), possibly with an additional class to manage several at once.

Third, the TweenFunction enum appears to exist only as a wrapper for a TweenEquation that can't be instantiated, and removing it in favor of the underlying TweenFunctions would make the API simpler and more flexible.

It would also be useful to have an event listener for when a tween finishes.

I'm sure I could find more ways to improve this API, but I haven't looked at it all that closely yet. Expect a PR shortly containing my proposed changes.

[nightm4re94]

You're very welcome to enhance this, it's been a bit of a rushed PR as we wanted to have a common ground for our project this month without overcomplicating the branching of the engine.
Some further changes have already been proposed in #353, please track your proposals there.