More `Rule`s.

[nicopap]

cuicui_layout is a bureaucracy and thrives on Rules. An utopia of Rules if you will…

I suggest three new rules. Those are missing pieces that lumex already has:

• Rule::Content(f32): Similar to LeafRule::Content, but generalized to containers.
• {Rule,LeafRule}::AspectRatio(f32): Get the size of this axis relative to the other, such that the aspect ration is preserved
• {Rule,LeafRule}::ContentAspectRatio: Get the size relative to the other, such as the aspect ratio is preserved. But the aspect ratio is inherited from the content.
• This would require dropping the content-size feature that keeps the aspect ratio. Dropping the feature will clarify where the size comes from.
• Convert ScreenRoot to use Content rule, since now we can use it for container nodes