Ask user for target card if multiple cards are suitable for key to card

Previously, always the first card was taken (under the assumption that
it's the only inserted card).

CardCommand::Private:
* Add possibility for subclasses to set the serial number

KeyToCardCommand:
* Remove superfluous supported() (always returned true)
* Add static method for getting the suitable cards for a given subkey
* Ask the user which card to write the key to if multiple suitable cards
  are available

SubKeysWidget:
* Always show the "Transfer to smartcard" option if the secret subkey
  is available; disable the option if no suitable cards are available

smartcard/utils.*, view/smartcardwidget.cpp:
* Make displayAppName() available everywhere

GnuPG-bug-id: 5066