Themes: making transitions easy.

[paulovbettio]

Themes should have a built-in method to tween values between StyleBoxes using a custom Curve, like CSS transition tag.

For example: we have a Button. This button have three StyleBoxFlat for its states, red when normal, pink when hovered and white when pressed. If we enable the transition on this Button class, the StyleBoxes will smoothly transition it's values to the values of the StyleBox of next state, so red will transition to pink when hovered, instead of just swapping states without animation.

[Calinou]

Morphing all of a StyleBoxFlat's properties over time is likely possible, but it will require making the StyleBoxFlat instance unique on a per-button basis.

[paulovbettio]

One approach could be using an instantiated style box for the transition and then replacing it back when the transition ends.

[acgc99]

What I do for this is taking a screenshot and animating (Tween) modulate.a of a TextureRect holding that screenshot. It gives nice results.

[Calinou]

Closing in favor of #8992, which is more detailed.