The problem was that the final export lacked the "#" prefix in the
same-document reference names. Fix this by doing what the ODF export
does: don't let the signature informations (data model) contain the "#",
just prepend it right before writing the name out.

This way it won't matter if the source of the name is an import (which
doesn't keep the "#") or a new signature (which did, previously), we'll
always write exactly one "#".

Change-Id: I18b6a5ba55b7e79ace537b7ecf575a7abc71e281