fdo#73487, fdo#73886: Overhaul cell borders to make them look sane.

Change-Id: Ie563f272b60ec8b6b8a4ff0df7256902997610c1
Reviewed-on: https://gerrit.libreoffice.org/7597
Reviewed-by: Andras Timar <andras.timar@collabora.com>
Tested-by: Andras Timar <andras.timar@collabora.com>