look for places where we are appending the temporary result of adding
strings together, to an OUStringBuffer, where we could rather call
append repeatedly and avoid the temporary creation

Change-Id: I481435124291ac7fb54b91a78344a9fe5b379a82
Reviewed-on: https://gerrit.libreoffice.org/59708
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>