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: 4794