We kept a backpointer from the Menu to its containing MenuBar, but it
was not used for anything so let's remove it.
Since all menus have the same font now, there's no need to track a
separate "title font".
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.
