Add support for replacing existing keys on PIV smartcards

CardCommand:

• Add serialNumber; this allows CardCommand to retrieve the card it is operating on

PIVGenerateCardKeyCommand:

• Add possibility to generate keys for non-empty key slots on the card overwriting the existing keys; ask the user for approval

PIVCard:

• Replace specific *KeyGrip() methods with keyGrip(keyRef)
• Add *KeyRef() methods for retrieving internal keyRef for the four main PIV keys

ReaderStatus:

• Add getCard() for retrieving a specific card by serial number

PIVCardWidget:

• Store card serial number
• Remove unused mCardIsEmpty

• GnuPG-bug-id: T4794