[AddressBook] Allow for incremental import

[cryptoBeliever]

Currently, it's possible to import the address book only if it's empty.
When at least one contact is added it's not possible to import from the file. Address book should allow for incremental import.

[cryptoBeliever]

@coiki could you look at this? If this is to do for next release?

[coiki]

@cryptoBeliever thank you. I checked both tasks and the other task, #1826 has a higher priority than this one, although both should be added to the next release.

[cryptoBeliever]

@OlegMakarenko do you think we should import in an incremental way as is now by overwriting?