so we can avoid temporary copies when appending a substring of an
OUString to the buffer. I would have preferred to call the method just
"append" but that results in ambiguous method errors when the callsite
is something like
   sal_Int32 n;
   OUStringBuffer s;
   s.append(n, 10);
I'm not sure why

Change-Id: I6b5b6641fcb5b26ce2269f89ef06e03c0b6aa76f
Reviewed-on: https://gerrit.libreoffice.org/58666
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>