Change OUStringBuffer::append() to operator+.

Change-Id: I13232cb69b1d62acfcf15d5ba5a2d72e5ab5d253
Reviewed-on: https://gerrit.libreoffice.org/44703Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Mark Hung <marklh9@gmail.com>
Reviewed-by: Michael Stahl <mstahl@redhat.com>