CollapsingState Improvements

[4JX]

As per #1566

Concerns are the FnOnce > Fn change, which shouldn't realistically impact any icon drawing functions and the ergonomics of a "no collapser" mode, since something like 4JX@f3234a1 would need to be done.

[4JX]

Would like to also add the commit mentioned above to this PR as well if it isn't deemed that much of an issue

[emilk]

what does it mean to match the indentation in this case?

[emilk]

My intent for CollapsingState was to make it more of a building block, and not something you customize. I would rather see a solution that looks something like this (from a users perspective):

let mut collapsing_state = …;
let header_response = ui.horizontal(|ui| {
    ui.label("Header");
    collapsing_state.show_toggle_button(ui, my_icon_painter);
});
collapsing_state.show_body(&header_response.response, ui, |ui| {
    ui.label("Body");
});

This keeps CollapsingState simple, and makes for a much more powerful and flexible solution.

[4JX]

This keeps CollapsingState simple, and makes for a much more powerful and flexible solution.

Oh, I was unaware a state could be constructed with an arbitrary response/header, hence the approach of this PR. I'll close it and draft a new one with the approach you described, as it seems a lot more useful.