fdo#74112 Preserve Chart displayunits properties during ODF import/export.

Change-Id: I38508b71ee5b3b966e1d36b0d253304fe17c0ec0
Reviewed-on: https://gerrit.libreoffice.org/7721
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Tested-by: Markus Mohrhard <markus.mohrhard@googlemail.com>