set undo buffer size, and mask ctl-z for when the undo operation is unavailable

svn: r1610

collects/drscheme/private/unit.ss

@@ -563,7 +563,10 @@ module browser threading seems wrong.
             
             (field [error-arrows #f])
             
-            (super-new))))
+            (super-new)
+
+	    (inherit set-max-undo-history)
+	    (set-max-undo-history 5000))))
       
       
                                                        

collects/framework/private/keymap.ss

@@ -989,6 +989,11 @@
 	      (map-meta "c:g" "ring-bell")
 	      (map "c:x;c:g" "ring-bell")
 	      (map "c:c;c:g" "ring-bell")
+
+              ;; This mapping is here to avoid having ctl-z insert "z"
+              ;;  when the "Undo" menu item is disabled, because inserting
+              ;;  "z" empties the redo stack
+              (map "c:z" "ring-bell")
 	      
 	      (map-meta "(" "insert-()-pair")
 	      (map-meta "[" "insert-[]-pair")