racket/gui/base: fix menu-bar%' get-frame' to return 'root

Return 'root istead of exposing the (hidden) root frame for
a menu bar that is shown when no frames are visible.

original commit: d0cfc34a77c23c8b5fd61b341dc6c446c3deae96

collects/mred/private/mrmenu.rkt

@@ -441,7 +441,7 @@
     (define wx-parent #f)
     (define shown? #f)
     (public*
-     [get-frame (lambda () prnt)]
+     [get-frame (lambda () (if (eq? root-menu-frame prnt) 'root prnt))]
      [get-items (entry-point (lambda () (send wx get-items)))]
      [enable (entry-point (lambda (on?) (send wx enable-all on?)))]
      [is-enabled? (entry-point (lambda () (send wx all-enabled?)))]

collects/scribblings/gui/menu-bar-class.scrbl

@@ -41,9 +41,10 @@ Enables or disables the menu bar (i.e., all of its menus).  Each
 
 
 @defmethod[(get-frame)
-           (is-a?/c frame%)]{
+           (or/c (is-a?/c frame%) 'root)]{
 
-Returns the menu bar's frame.
+Returns the menu bar's frame, or returns @racket['root] if the menu
+bar is shown when no other frames are shown.
 
 }