idea from tml.

Extend the unusedfields plugin to find fields that are only assigned in
the constructor.

Change-Id: I258d3581afbe651d53ce730c9ba27a4598cd9248
Reviewed-on: https://gerrit.libreoffice.org/57733
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>