Fix so wxMDIParentFrame is actually shown when Show(TRUE) is called.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_2_2_BRANCH@7560 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/msw/mdi.cpp

@@ -222,6 +222,9 @@ bool wxMDIParentFrame::Create(wxWindow *parent,
 
   wxModelessWindows.Append(this);
 
+  // unlike (almost?) all other windows, frames are created hidden
+  m_isShown = FALSE;
+
   return TRUE;
 }