look for fields which are only assigned to in the constructor, so they
can be made const

Change-Id: I0b76817c2181227b04f6a29d6a808f5e31999765
Reviewed-on: https://gerrit.libreoffice.org/60393
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>