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: T5066