* Tweens were changed from Node to RefCounted. New API is inspired by DOTween.
* Tweens are created and managed by SceneTree, similar to SceneTreeTimer, which makes them ultra cheap to use a lot.
* Animating with Tweens is done by creating sequences of Tweeners. You create them from code and they autostart by default (fire-and-forget).
* There are 4 Tweeners that cover the former Tween functionality: PropertyTweener, IntervalTweener, CallbackTweener and MethodTweener.
* The methods were simplified a lot. Long argument lists are replaced with chained calls on Tweens and Tweeners.
* Tweeners by default execute in sequence, so it's easy to create complex chained animations.
* You can bind a Tween to a node. Tween will be removed automatically when the bound node is freed.
It was Godot-ified and integrated into Tween originally (#628)
without mention that it was thirdparty code, but it's actually
derived from https://github.com/jesusgollonet/ofpennereasing.
It's also very bad quality code and should be replaced by a better,
properly-maintained library of easing equations.
