pub trait ContainerImplExt: ObjectSubclass {
    fn parent_add(&self, container: &Self::Type, widget: &Widget);
    fn parent_remove(&self, container: &Self::Type, widget: &Widget);
    fn parent_check_resize(&self, container: &Self::Type);
    fn parent_set_focus_child(
        &self, 
        container: &Self::Type, 
        widget: Option<&Widget>
    );
    fn parent_child_type(&self, container: &Self::Type) -> Type;
    fn parent_path_for_child(
        &self, 
        container: &Self::Type, 
        widget: &Widget
    ) -> WidgetPath;
    fn parent_forall(
        &self, 
        container: &Self::Type, 
        include_internals: bool, 
        callback: &Callback
    );
}