improve editor-canvas resizing

A recent bug fix involved moving part of an `editor-canvas%' resize
out of atomic mode by queueing a callback (because the resize involves
quesrying the editor for its size, etc.), but then the callback
happens after a canvas is shown, which can cause it to appear with
bogus initial scrollbars. Queue the callback instead as a "refresh"
level callback, which gets a chance to run before a frame is made
visible.

collects/mred/private/kernel.rkt

@@ -30,6 +30,7 @@
          eventspace-handler-thread
          eventspace-event-evt
          queue-callback
+         queue-refresh-event
          middle-queue-key
          get-top-level-windows
          begin-busy-cursor

collects/mred/private/wxme/editor-canvas.rkt

@@ -8,7 +8,7 @@
          racket/snip/private/prefs
          racket/snip/private/private
          (only-in "cycle.rkt" popup-menu%)
-         (only-in "../helper.rkt" queue-window-callback)
+         (only-in "../kernel.rkt" queue-refresh-event)
          "wx.rkt")
 
 (provide editor-canvas%)
@@ -292,8 +292,8 @@
   
   (define/override (on-size)
     (unless noloop?
-      (queue-window-callback
-       this
+      (queue-refresh-event
+       (get-eventspace)
        (lambda ()
          (unless (and media
                       (send media get-printing))