Export the Overlap value with 100% for stacked charts, because the
default overlap value of the Bar/Column chart is 0% and LibreOffice
do nothing with the overlap value in Stacked Chart case, unlike the
MS Office.

Change-Id: If4e20b88c2b1180f68a8d2b610c407d674a8498b
Reviewed-on: https://gerrit.libreoffice.org/59448Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Tested-by: Jenkins