add GetMDIParent() and use it to reduce the number of casts in the MDI code (patch 1885462)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@51622 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/msw/mdi.h

@@ -162,6 +162,11 @@ public:
     // Implementation only from now on
     // -------------------------------
 
+    wxMDIParentFrame* GetMDIParent() const
+    {
+        return wxStaticCast(wxFrame::GetParent(), wxMDIParentFrame);
+    }
+
     // Handlers
     bool HandleMDIActivate(long bActivate, WXHWND, WXHWND);
     bool HandleWindowPosChanging(void *lpPos);