Removed some chained OUString::append()'s from forms. OUStringBuffer
could really use a append operator(+=).

Change-Id: I635bdd25da9f09373e686d87a1d198bc09bda906
Reviewed-on: https://gerrit.libreoffice.org/5329Reviewed-by: Norbert Thiebaud <nthiebaud@gmail.com>
Tested-by: Norbert Thiebaud <nthiebaud@gmail.com>