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

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

src/msw/frame.cpp

@@ -807,7 +807,7 @@ void wxFrame::IconizeChildFrames(bool bIconize)
         // the child MDI frames are a special case and should not be touched by
         // the parent frame - instead, they are managed by the user
         wxFrame *frame = wxDynamicCast(win, wxFrame);
-        if ( frame && !wxDynamicCast(frame, wxMDIChildFrame) )
+        if ( frame && !frame->IsMDIChild() )
         {
             frame->Iconize(bIconize);
         }