would have preferred to re-use o3tl::strong_int, of which this
is a modified copy, but there are lots of convenience accessors
which are nice to define on the class.

Change-Id: I301b807aaf02fbced3bf75de1e1692cde6c0340a
Reviewed-on: https://gerrit.libreoffice.org/38497Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>