newaccountmodel: add the ability to have a different avatar by account

In NewAccountModel: add a method to change the avatar of an account.

Note: We still use the old LRC (call.cpp) to send VCard during a call,
so sendProfile should be able to get the avatar from the database if
possible.

Change-Id: I74bc6d8aea64aa3bdfbf49d2f353f2f9b92325c1
Reviewed-by: Hugo Lefeuvre <hugo.lefeuvre@savoirfairelinux.com>