Add `Response::closed` and investigate generalizing `popup::OpenKind` as something that could be used with `Window`, `Collapsing`, `Area` etc...

See https://github.com/emilk/egui/pull/5713#discussion_r1959481851

https://github.com/emilk/egui/pull/5713#discussion_r1959485606