add keyboard shortcuts to unsaved dialog warning on mac os x

closes PR 12927
closes PR 10708
closes PR 8996

original commit: 64fd649ee573c73499566b7cbb77271842a33ab7

pkgs/gui-pkgs/gui-lib/framework/gui-utils.rkt

@@ -230,9 +230,30 @@
           (λ () (thunk))
           (λ () (cursor-off))))])))
 
-(define unsaved-warning
-  (lambda (filename action-anyway (can-save-now? #f) (parent #f) [cancel? #t])
-    (let ([mb-res (message-box/custom 
+(define (unsaved-warning filename action-anyway [can-save-now? #f] [parent #f] [cancel? #t])
+  (define key-closed #f)
+  (define (unsaved-warning-mixin %)
+    (class %
+      (inherit show)
+      (define/override (on-subwindow-char receiver evt)
+        (define (is-menu-key? char)
+          (and (send evt get-meta-down)
+               (equal? (send evt get-key-code) char)))
+        (cond
+          [(is-menu-key? #\d)
+           (set! key-closed 'continue)
+           (show #f)]
+          [(is-menu-key? #\s)
+           (set! key-closed 'save)
+           (show #f)]
+          [(is-menu-key? #\c)
+           (set! key-closed 'cancel)
+           (show #f)]
+          [else
+           (super on-subwindow-char receiver evt)]))
+      (super-new)))
+  (define mb-res 
+    (message-box/custom 
      (string-constant warning)
      (format (string-constant file-is-not-saved) filename)
      (string-constant save)
@@ -242,11 +263,15 @@
      (if can-save-now?
          '(default=1 caution)
          '(default=2 caution))
-                   2)])
+     2
+     #:dialog-mixin (if (equal? (system-type) 'macosx)
+                        unsaved-warning-mixin
+                        values)))
+  (or key-closed
       (case mb-res
         [(1) 'save]
         [(2) 'cancel]
-        [(3) 'continue]))))
+        [(3) 'continue])))
 
 (define get-choice
   (lambda (message