improve the plugin to find fields which are only assigned to in the
constructor

Change-Id: I95b5be238ebba83d950ca15093abdd1849740359
Reviewed-on: https://gerrit.libreoffice.org/35613Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>