just use OUStringBuffer here, and consequently avoid re-implementing
such string-buffer handling code

Change-Id: I61e39dada6f46478b9d289f0310bb6846eb9868b
Reviewed-on: https://gerrit.libreoffice.org/62646
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>