This patch adds two new API's:
- WidgetType& GUI::Window::set_main_widget<WidgetType>();
This creates a new main widget for a window, assigns it, and returns
it to you as a WidgetType&.
- LayoutType& GUI::Widget::set_layout<LayoutType>();
Same basic idea, creates a new layout, assigns it, and returns it to
you as a LayoutType&.
03e0ddce52