added wxFrame::IsMDIChild() (msw only, protected)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@8820 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/msw/frame.h

@@ -141,6 +141,8 @@ protected:
 
     // window proc for the frames
     long MSWWindowProc(WXUINT message, WXWPARAM wParam, WXLPARAM lParam);
+    
+    virtual bool IsMDIChild() const { return FALSE; }
 
     bool                  m_iconized;
     WXHICON               m_defaultIcon;