fdo#74112  Write Chart Displayunits information back to XML.

   During export access properties stored during import
   and write back those. Currently we just support basic chart
   display unit information such as builtinunit, there are more
   properties ex. custUnit, dispUnitsLbl, extLst.
   which are pending.

Conflicts:

	chart2/qa/extras/chart2export.cxx

Change-Id: I87e0a8322fc0b0c3df2b8053b7f29fffea611040
Reviewed-on: https://gerrit.libreoffice.org/7722
Tested-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>