replace m_insertCallback with a virtual function, contrary to the old comments a virtual works just fine

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@55354 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/gtk/mdi.h

@@ -183,6 +183,8 @@ public:
     virtual bool CreateClient( wxMDIParentFrame *parent, long style = wxVSCROLL | wxHSCROLL );
 
 private:
+    virtual void AddChildGTK(wxWindowGTK* child);
+
     DECLARE_DYNAMIC_CLASS(wxMDIClientWindow)
 };