This makes it easier to spot syntax errors when editing the
class reference. The schema is referenced locally so validation
can still work offline.
Each class XML's schema conformance is also checked on GitHub Actions.
For the time being we don't support writing a description for those, preferring
having all details in the method's description.
Using self-closing tags saves half the lines, and prevents contributors from
thinking that they should write the argument or return documentation there.
* 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.
