tdf#126193 Chart OOXML: Export Multi-level category labels
Fix export of Multi-level category axis labels with the correct
OOXML tags (as the OOXML Standard requested) in the correct
order.
Also fix tdf#126195: but only the export part of the whole fault,
which combined (united) the text of the category axis labels at
different levels.
Change-Id: Iefcef00818a3bb2ee1671bf693335904be471722
Reviewed-on: https://gerrit.libreoffice.org/75299
Reviewed-by: László Németh <nemeth@numbertext.org>
Tested-by: László Németh <nemeth@numbertext.org>