Fix CPPopUpButton menu support.

- Added enumerateObjectsUsingBlock: to CPArray
- Fixed a remaining case of [] being used instead of objectAtIndex: in CPMutableArray.
- Added KVO support for menu items array in menus.
- Changed CPPopUpButton to use KVO to sync up with menu instead of notifications.
- Removed extra menu encoding in CPPopUpButton.
- Removed extra mainMenu object from CPApplication.
- Use setters in CPResponder instead of direct ivar access in initWithCoder:.

Closes #298.
Closes $1059.

Reviewed by me.

Foundation/CPArray/CPArray.j

@@ -517,6 +517,14 @@ var concat = Array.prototype.concat,
             objj_msgSend([self objectAtIndex:index], aSelector);
 }
 
+- (void)enumerateObjectsUsingBlock:(Function)aFunction
+{
+    var index = 0,
+        count = [self count];
+
+    for (; index < count; ++index)
+        aFunction([self objectAtIndex:index], index);
+}
 
 // Comparing arrays
 /*!