cocoa: fix focus method

Don't confuse "should the Tab key advance the focus to this window?"
with "can this window have the focus?".

gui-lib/mred/private/wx/cocoa/canvas.rkt

@@ -814,7 +814,7 @@
                 (not (send e button-down? 'left))
                 (not (on-menu-click? e)))))
 
-     (define/override (gets-focus?)
+     (define/override (can-accept-focus?)
        wants-focus?)
      (define/override (can-be-responder?)
        (and wants-focus? (is-enabled-to-root?)))

gui-lib/mred/private/wx/cocoa/item.rkt

@@ -44,7 +44,7 @@
     (let ([on? (and on? (is-window-enabled?))])
       (tellv (get-cocoa-control) setEnabled: #:type _BOOL on?)))
 
-  (define/override (gets-focus?)
+  (define/override (can-accept-focus?)
     (tell #:type _BOOL (get-cocoa-control) canBecomeKeyView))
 
   (define/public (command e)

gui-lib/mred/private/wx/cocoa/message.rkt

@@ -117,7 +117,7 @@
      [else
       (tellv (get-cocoa) setImage: (bitmap->image label))]))
 
-  (define/override (gets-focus?) #f)
+  (define/override (can-accept-focus?) #f)
 
   (define/public (set-preferred-size)
     (tellv (get-cocoa) sizeToFit)

gui-lib/mred/private/wx/cocoa/tab-panel.rkt

@@ -206,7 +206,7 @@
       (when control-cocoa
         (tellv control-cocoa setEnabled: #:type _BOOL on?))))
 
-  (define/override (gets-focus?)
+  (define/override (can-accept-focus?)
     (and (not control-cocoa)
          (tell #:type _BOOL tabv-cocoa canBecomeKeyView)))
   (define/override (get-cocoa-focus)

gui-lib/mred/private/wx/cocoa/window.rkt

@@ -751,7 +751,7 @@
         (accept-drags-everywhere (or accept-drag? accept-parent-drag?))))
 
     (define/public (set-focus)
-      (when (and (gets-focus?)
+      (when (and (can-accept-focus?)
                  (is-enabled-to-root?))
         (let ([w (tell cocoa window)])
           (when w
@@ -912,7 +912,8 @@
             (tellv content addCursorRect: #:type _NSRect r cursor: cursor-handle)))))
     (define/public (get-cursor-width-delta) 0)
     
-    (define/public (gets-focus?) #f)
+    (define/public (can-accept-focus?) #f)
+    (define/public (gets-focus?) (can-accept-focus?))
     (define/public (can-be-responder?) (is-enabled-to-root?))
 
     (define/public (on-color-change)