feat: Implementation of basic box-model widgets.

[xanahopper]

Widgets

• ConstrainedBox
• UncontrainedBox
• ColoredBox
• SizedBox

Traits

• BoxLayoutWidget, common for box-model measure and layout
• ChildWidget, widgets with children.

[toiglak]

Sorry to keep you waiting.

You added the BoxLayoutWidget trait, but we lay out widgets using “box constraints'' by default in Frui. You can see this in the layout method of RenderWidget which takes the Constraints argument – the same as in RenderBox in Flutter.

As for the compute_max_intrinsic_width method and the similar, I think we can add them directly to the RenderWidget. Alternatively (and I really would like to try this option out) we could implement the measure method from Xilem, which seems to do the exact thing we need here!

[xanahopper]

Yes, adding toRenderWidget directly and splitting the layout into measure and layout are considered.
BoxLayoutWidget is declared because I want to keep a position for the Scrolling/Sliver model, but now it seems unnecessary.

And I'm trying some patterns to simulate the OOP abstraction design and defined some default behaviors that can be overridden, but I think it's not suitable for Rust.

I'll change these directly to RenderWidget later in this PR.

[xanahopper]

@toiglak BoxLayoutWidget has been removed, and methods in this move to RenderWidget with the default implementation.

[xanahopper]

Flex PR #9 is based on this.