[racket] Can I make the frame% instances transparent?

The `frame%` class does not offer a transparency option, but on Mac OS
X, you can use the FFI to make a frame transparent at the Cocoa
NSWindow layer.

Here's an example:

--------------------------------------------------

#lang racket/gui
(require ffi/unsafe
         ffi/unsafe/objc)

;; Create the frame:
(define f (new frame%
               [label "Example"]
               [width 200]
               [height 200]))

;; Add a transparent canvas that displays "Hello"
(new canvas%
     [parent f]
     [style '(transparent)]
     [paint-callback (lambda (c dc)
                       (send dc draw-text "Hello" 0 0))])

;; The `CGFloat` type depends on the platform:
(define 64-bit? (= (ctype-sizeof _long) 8))
(define _CGFloat (if 64-bit? _double _float))

;; Get an `NSWindow` from the `frame%` object:
(define h (send f get-handle))

;; Set the background's transparency at the `NSWindow` level:
(import-class NSColor)
(tellv h setBackgroundColor: (tell NSColor 
                                   colorWithRed: #:type _CGFloat 1.0
                                   green: #:type _CGFloat 1.0
                                   blue: #:type _CGFloat 1.0
                                   alpha: #:type _CGFloat 0.5))
(tellv h setOpaque: #:type _BOOL NO)

;; Show the frame:
(send f show #t)