Apply the same workaround as was applied in r67771 to wxMessageDialog to
wxRichMessageDialog and map IDCANCEL returned from it to IDOK if "Cancel" is
the only button in the dialog -- as in this case it's really just a renamed
(for convenience of allowing the user to use "Esc" to close it) "OK".
Closes#16153.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76257 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775