fdo#72520 : Added property to store external data path in chart.

  -  added new property entry to store xlsx external data for chart.

Change-Id: I3695865ad7e888359d1048e9bc380cba26108636
Reviewed-on: https://gerrit.libreoffice.org/7212
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Tested-by: Markus Mohrhard <markus.mohrhard@googlemail.com>