gtk menu-bar fixes

collects/mred/private/wx/gtk/menu-bar.rkt

@@ -114,9 +114,11 @@
 	(gtk_label_set_text_with_mnemonic (gtk_bin_get_child item-gtk) 
                                           (fixup-mneumonic str)))))
     
-  (def/public-unimplemented enable-top)
+  (define/public (enable-top pos on?)
+    (gtk_widget_set_sensitive (car (list-ref menus pos)) on?))
 
   (define/public (delete which pos)
+    (atomically
      (set! menus (let loop ([menus menus]
                             [pos pos])
                    (cond
@@ -127,7 +129,7 @@
                      (cdr menus)]
                     [else (cons (car menus)
                                 (loop (cdr menus)
-                                     pos))]))))
+                                      (sub1 pos)))])))))
 
   (public [append-menu append])
   (define (append-menu menu title)