Layouts: change the representation of layout trees

[Antikyth]

Working with layouts is complicated a lot by my representation of the layout tree. While it is the most obvious way to represent a tree, I understand now that there are other ways, and ways that will make other layout features that need to be implemented easier.

The main complication with these layouts is that each group has a lot more information than just its children (orientation, dimensions, coordinates at minimum).

I think a better approach here, which I haven't learned that much about yet, is to somehow use Rc and Weak to let nodes have references to their parents. I'm not sure how mutating groups' orientation, dimensions, and coordinates fits in with that.