added revert to defaults

svn: r5575

collects/framework/private/preferences.ss

@@ -238,6 +238,12 @@ the state transitions / contracts are:
              [cancel-callback (λ (_1 _2)
                                 (hide-dialog)
                                 (preferences:restore-prefs-snapshot stashed-prefs))])
+      (new button%
+           [label (string-constant revert-to-defaults)]
+           [callback
+            (λ (a b)
+              (preferences:restore-defaults))]
+           [parent bottom-panel])
       (gui-utils:ok/cancel-buttons
        bottom-panel
        ok-callback

collects/string-constants/english-string-constants.ss

@@ -418,7 +418,8 @@ please adhere to these guidelines:
  (background-color "Background Color")
  (default-text-color "Default text") ;; used for configuring colors, but doesn't need the word "color"
  (choose-a-background-color "Please choose a background color")
-
+ (revert-to-defaults "Revert to defaults")
+  
  ; title of the color choosing dialog
 
  ; should have entire alphabet