Make CntUnencodedStringItem sortable, implementing operator<.
This takes the load time from 4s to 3.3s

Change-Id: I532cdf65149a733d41d2caf367675800d2ba4d41
Reviewed-on: https://gerrit.libreoffice.org/71460
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>