Fix return value of wxRichMessageDialog with only an "OK" button.

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/branches/WX_3_0_BRANCH@76254 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775