state: Select the layout

This change cleans up some more of the old C/Rust spaghetti regarding layout selection. It's now concentrated in state.rs, and the arrangement kind selection (base/wide) is no longer duplicated in C code.

As a downside, there's a noodle threading all the way to the state of the popover, and it may be difficult to get rid of.

All in all, this moves panel logic closer together, strips the C code of responsibility, and sets the stage for adjusting panel size in the layout, which I count as a slight win against the 100 extra lines of code.