Checkbox widget implementation

[presiyan-ivanov]

Implementation suggestions:
The API should look something like this:

fn app_view(cx: AppContext) -> impl View {
    let (is_checked, set_is_checked) = create_signal(cx.scope, false);
    click(
        cx,
        |cx| {
            stack(cx, |cx| {
                (
                    check_box(cx, move || is_checked.get()),
                    label(cx, || "Is checked?".to_string()),
                )
            })
        },
        move || set_is_checked.update(|checked| *checked = !*checked),
    )
}

The disabled fn in decorator.rs can be used as a reference to implement the checked fn of check_box.
The check_box should simply render a container with a border that has an svg for the check mark.

Bonus points if the checkbox is added to the widget-gallery example :)