Fix SIGSEGV due bad pointer in wxMessageDialog (wxQT)

m_qtWindow should be used instead of m_qtMessageBox (removed). If not, PostCreation() cannot call wxMessageDialog::GetHandle() as it is virtual (and it is called from the ctor), so it fails to set the base window pointer, raising a SIGSEGV in wxWindow::DoSetSize (for more info, see architecture in docs)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@78312 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775