Support transferring X.509 signing certificates to PIV cards

* Move file-static keyDisplayName() to PIVCard to make it available
outside of PIVCardWidget.
* Transfer signing certificate to PIV card asking for card slot if
multiple card slots match the certificate usage.
* Ensure that public key of certificate to transfer matches public key
on card.
* Suppress overwrite warning if key on card matches key to transfer
to card.

GnuPG-bug-id: 4794