sw: paragraph-sign: process signatures in one pass on loading
This combines the metadata field restoration logic with
validation/update of said field in one pass upon loading docs.
This cuts down overheads and makes the code more readable.
In addition, now that paragraph signature validation is quite
cheap (separate commits), esp. when no signatures exist, we
validate and update signatures (where applicable) when fields
are updated.
Change-Id: I4adcea579c591f9be457ed742d2cf54fa308163d
Reviewed-on: https://gerrit.libreoffice.org/63008
Tested-by: Jenkins
Reviewed-by: Ashod Nakashian <ashnakash@gmail.com>