Look up certificates for NetKey cards in widget instead of card

This makes it possible to update the list of certificates (e.g. after
the card certificates have been learned) without having to trigger a full
update of the card information.
And for other card types the certificate lookup is also done in the widget.

(cherry picked from commit 88193b80256d62332a6e6ad5fb422c2f5a6c0743)

• GnuPG-bug-id: T6846