Consistently use it for boxed lists in inspector and gtk-demo.
Fixes issues like #3145 (moved)
Marking as draft because when discussing it in #gtk I forgot another thing: separators. For boxed lists you always want to have separators, and the old method relied on adding them manually - e.g. half of the inspector lists were missing them.
If we don't want that, another option is .frame that doesn't touch separators.