Make child's parent fit the child, in new_child

[JerzySpendel]

Is your feature request related to a problem? Please describe.
I use bevy and bevy_egui libraries. I have this approach that my widget-system-functions receive as an input a part of Ui (owned Ui struct) and add widgets on it. The only way to pass Ui part to the bevy system function that I could figure out, was to create an owned Ui structure by new_child and insert it to entity for later execution. The problem is that when creating an owned version of Ui I have to allocate the space manually before, for example:

        egui::Window::new("Adjust CPU Usage")       
            .open(&mut window_open.0)
            .max_size(egui::Vec2::new(300.0, 100.0))
            .show(ctx, |ui| {
                let response = ui.allocate_space(ui.available_size());
                let child_ui = ui.new_child(
                    UiBuilder::default()
                        .max_rect(egui::Rect::from_min_size(response.1.min, response.1.size())),
                );

                ui_callbacks.push(UiCallback {
                    system_id: programs_cpu_usage_system_id,
                    ui: child_ui,
                });
            });

The problem here is that in fact only a fraction of the original allocated space might get used, leaving the rest of allocated space unusable and oftentimes the parent ui too big.

Describe the solution you'd like
I would expect new_child to work similarly to for example allocate_new_ui where the parent Ui will shrink to match the size of child ui.

Describe alternatives you've considered
I considered using on of: allocate_new_ui, allocate_ui but they expect callback function. In my design I want my widget function to also be bevy systems, that look something like this:

fn total_cpu_usage(In(mut ui): In<Ui>, programs: Query<&Program>, cpu: Query<&Cpu>) {
    let total_usage = programs
        .iter()
        .map(|program| program.cpu_usage)
        .sum::<f32>();

    let bar = egui::ProgressBar::new(total_usage).show_percentage();
    ui.label(format!(
        "CPU ({:.2} GHz): ",
        cpu.get_single().unwrap().frequency
    ));
    ui.add(bar);
}

and this architecture actually works, except for the flaw that is described in first point.