Persist first phase registration parameters

[daniele-athome]

This can be seen as a bug since one might go back and restart registration endlessly :-)
Persist into preferences registration parameters from phase one:

• server used for registration
• phone number
• name
• key pair

This way, reopening Kontalk will direct the user to the code verification screen. After that, user would also need a button to go back (use a cancel button or the home-as-up button?).

[daniele-athome]

Under way on branch persist_registration.

[daniele-athome]

Key storing and restoring seems to be ok. However, some real tests should be done, especially for existing keys import.

[daniele-athome]

Continuing on next beta.