Approach for hidden Stylebox animations

[dugramen]

I'm working on a PR for stylebox animations. It works by providing an animation id when drawing a stylebox, and transitioning its properties anytime a different box is drawn with that id

This works for all transitions from normal, hover, pressed etc. But it does not work for things that don't have a "default" box, rather they just don't draw anything. For example ItemList items, Tree items, MenuBar buttons, TabBar buttons, and all focus styles.

There are 2 solutions I have in mind:

• Add "normal" StyleBox theme items for all those that need it, including a "focus_hidden" for animating out focus. By default they can be StyleBoxEmpty to keep everything looking & performing the same

• Add an "invisible" or "hidden" or "exiting" property to each stylebox, which is a nested StyleBox. When nothing is drawn for a given animation id that frame, it will instead use the last drawn stylebox and animate it to its nested hidden stylebox

The 1st version might raise a bit of confusion with "focus_hidden", and it would add a bunch more styles. The 2nd version might be even more confusing all around, but it'd have higher compatibility.

I personally think the 1st option is better, but I wanted to get feedback first

[dugramen]

Actually scrap this. I have a slightly better idea I'll open a proposal for