This patch begins the transition away from the global menu towards
per-window menus instead.
The global menu looks neat, but has always felt clunky, and there
are a number of usability problems with it, especially in programs
with multiple windows.
You can now call GUI::Window::set_menubar() to add a menubar to
your window. It will be specific to that one window only.
Also, only mark the menu bar item as opened if a menu was actually
opened through the menu bar.
These changes allow a menu to be used both in the menu bar as well
as a context menu.
Fixes#5469
