ffi/unsafe/objc: fix objc-is-a? to recognize subclass instances

This repair turns out to matter for Mac OS X 10.10, where creating
a list box in a frame somehow makes the frame an instance of a
subclass (that implements notifications of some sort, I think).
That subclassing broke an `(objc-is-a? ... RacketWindow)` test to
recognize windows that belong to a particular eventspace.

pkgs/racket-pkgs/racket-doc/scribblings/foreign/objc.scrbl

@@ -258,7 +258,16 @@ Returns a selector (of FFI type @racket[_SEL]) for the string form of
 @defproc[(objc-is-a? [obj _id] [cls _Class]) boolean?]{
 
 Check whether @racket[obj] is an instance of the Objective-C class
-@racket[cls].}
+@racket[cls] or a subclass.
+
+@history[#:changed "6.1.0.5" @elem{Recognize subclasses, instead of requiring an
+                                   exact class match.}]}
+
+@defproc[(objc-subclass? [subcls _Class] [cls _Class]) boolean?]{
+
+Check whether @racket[subcls] is @racket[cls] or a subclass.
+
+@history[#:added "6.1.0.5"]}
 
 @; ----------------------------------------------------------------------
 
@@ -289,6 +298,14 @@ Registers an Objective-C class.}
 
 Returns the class of an object (or the meta-class of a class).}
 
+@defproc[(class_getSuperclass [cls _Class])
+         _Class]{
+
+Returns the superclass of @racket[cls] or @racket[#f] if @racket[cls]
+has no superclass.
+
+@history[#:added "6.1.0.5"]}
+
 @defproc[(class_addMethod [cls _Class] [sel _SEL] 
                           [imp procedure?]
                           [type ctype?]

racket/collects/ffi/unsafe/objc.rkt

@@ -876,10 +876,19 @@
 
 ;; --------------------------------------------------
 
-(provide objc-is-a?)
+(provide objc-is-a?
+         objc-subclass?)
+
+(define-objc class_getSuperclass (_fun _Class -> _Class))
 
 (define (objc-is-a? v c)
-  (ptr-equal? (object-get-class v) c))
+  (objc-subclass? (object-get-class v) c))
+
+(define (objc-subclass? vc c)
+  (or (ptr-equal? vc c)
+      (let ([pc (class_getSuperclass vc)])
+        (and pc
+             (objc-subclass? pc c)))))
 
 ;; --------------------------------------------------