Include X.509 certificates under Tools->Options->UserData
among the keys for signing.
Add a new checkbox to Save file dialog, to sign with that
selected key easily.
The checkbox is disabled if there's no matching key found.
Change-Id: I9fc16790c479819cd1f35bcad040d0ebc7c4bdef
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/170619
Tested-by: Jenkins
Reviewed-by: Sarper Akdemir <sarper.akdemir@allotropia.de>
