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/mdi.h

@@ -174,6 +174,7 @@ protected:
     virtual void DoGetPosition(int *x, int *y) const;
     virtual void DoSetClientSize(int width, int height);
     virtual void InternalSetMenuBar();
+    virtual bool IsMDIChild() const { return TRUE; }
 
     // common part of all ctors
     void Init();