Add reparenting setParent function to GuiElement

[oznogon]

Add a GuiElement::setParent() function to reparent elements by removing them from their former parent element's tree, adding them to the new parent element's tree, and updating its pointer.

For example, this allows for the dynamic reparenting of elements based on the parent element's width or height.

    // If a container already exists for this system, reparent it to the new row instead of recreating widgets.
    if (auto this_container = systems[system_index].container)
        this_container->setParent(systems_row);
    ...