There was a lot of error handling here previously when writing to
a config file failed, but this was removed since we have no way of
conveying a `Config::write` failure from the ConfigServer.
Applications previously had to create a GUI::Menubar object, add menus
to it, and then call GUI::Window::set_menubar().
This patch introduces GUI::Window::add_menu() which creates the menubar
automatically and adds items to it. Application code becomes slightly
simpler as a result. :^)
Scoring is designed to mimic Microsoft's implementation - starting at
500, decreasing by 1 every move, and increasing by 100 for every full
stack.
Fixes GH-5319.
