Show UI for all inserted cards in tabs

ReaderStatus:
* Use more specific signals cardAdded, cardChanged, cardRemoved instead
  of just cardChanged.
* Send serial number and app name (which uniquely identify an application
  on a smartcard) with the signals

NetKeyWidget, PGPCardWidget, PIVCardWidget:
* Remove double margins around the widgets; this also avoids an ugly
  lightgray border around the widgets

SmartCardWidget:
* Use a QTabWidget with tabs for each application on the inserted
  smartcards

GnuPG-bug-id: 5066