menu-bar% object is created for a particular
frame% object. A frame can have at most one menu bar;
exn:application:mismatch exception is raised when a new menu bar is created for a frame that
already has a menu bar.
Creates a menu bar in the specified frame. The menu bar is initially
'root is supplied as
parent, the menu
bar becomes active only when no other frames are shown. A
parent is allowed only when
#t, and only if no such menu bar has been created before,
exn:application:mismatch exception is raised.
demand-callback procedure is called by the default
on-demand method with the object itself.