Avoid having "CARD 1" or "CARD 2" as carrier name.

* Each SIM card is now bound to a unique key pair.
* Display number or slot index if display name is the same for multiple
  cards.
* Use subcription ID of last received message as default SIM, but do not
  change it if user is in conversation.
* Update app subscription IDs on new phone
* Start secure session only for SIM which received message, allow user to
  select SIM when starting a secure session

Fixes #467
Fixes #497

Fixes #757

Fixes #757

But it can be disabled in Advanced Preferences.

Fixes #566

Now, there is only one screen (previous layout was full of glitches).