Fix centering of top-level children of wxMDIParentFrame on Mac

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@30706 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/mac/carbon/mdi.h

@@ -68,6 +68,8 @@ public:
 
   // Get the client window
   inline wxMDIClientWindow *GetClientWindow() const { return m_clientWindow; };
+  // Get rect to be used to center top-level children
+  virtual void GetRectForTopLevelChildren(int *x, int *y, int *w, int *h);
 
   // Create the client window class (don't Create the window,
   // just return a new class)