DOCX part was done in fb959e58.

This commit fixes DOC part. Line width wasn't taken into account on
import; and export was done only with "from text" distance, which
gave poor interoperability with Word, where the borders were close
to page edge.

The common code is moved to editeng/source/items/frmitems.cxx and
include/editeng/boxitem.hxx.

Change-Id: I3d1d1312cb9dc9a9e00d9847ec11234cd787df60
Reviewed-on: https://gerrit.libreoffice.org/51366Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>
Reviewed-on: https://gerrit.libreoffice.org/57704Tested-by: Mike Kaganski <mike.kaganski@collabora.com>
Reviewed-by: Jan Holesovsky <kendy@collabora.com>