https://wiki.openoffice.org/wiki/Writer/MSInteroperability/PageBorder
discusses implementation differences between ODF model and MS formats
wrt dealing with page margins and distances to borders.

This patch corrects import from DOCX, so that the border distance and
width doesn't add to the margin size imported from file anymore. It
takes care to preserve size from page edge to text (the most important
size that affects document layout). When borders go outside of range
valid for ODF, the margin is set to keep text area intact, and the
border is placed as close to intended position as possible.

Export code now also properly handles border width. Also, an improved
heuristic implemented to better export cases unsupported by Word, so
that the result would look closer to ODF original. We still write
correct sizes to OOXML, so that when reopened by LO, the borders will
be in correct places; but as Word cannot handle sizes more than 31 pt,
it will show borders shifted.

This prevents from adding border widths and distances to page margins
at each opening of DOCX, saving back the changed value, increasing
the margins each time.

Change-Id: Ia978ab119dd661949d6c321aea91397f28d205b0
Reviewed-on: https://gerrit.libreoffice.org/51267Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>