Multi-device support

- Add properties to Account:
    * deviceId:
        The id of the current device for this account
    * archivePassword:
        The password used to encrypt the account archive
        by the daemon
    * needsMigration:
        Whether or not this account needs to be migrated
        to the new archive format.
    * archivePin:
        The pin used to import the account from the Ring.

- Add new methods to Account:
    * exportOnRing:
        Initializes an account export operation, returns True or
        false depending on whether the transaction was initialized
        successfully or not. exportOnRingEnded is emitted on operation
        completion.

- Add a new model to Account:
      * RingDevicesModel:
          This models contains a list of known devices, their name,
          and their ID. For now, the name is just the short ID.
          Devices are added as they are discovered by the daemon.

- Add a new method to AccountModel
   * accountsToMigrate:
       This methods returns a list of accounts that need migration. To
       the new archive format.

Note: Migration to the new archive format is done like so:
  > account->setArchivePassword(password);
  > account->performAction(Account::EditAction::SAVE)

- Add new signals to AccountModel:
    * knownDevicesChanged:
        emitted when the list of known Ring devices has changed.
        For example, it is emitted when the account is imported on a
        new device.
    * exportOnRingEnded:
        emitted after the operation initialized with exportOnRing has
        completed. The signal contains the status of the operation and
        the pin if the operation was a success.

- Add new RegistrationState:
  * INITIALIZING:
      This state is used when the account is being created or imported
      from the Ring.

Change-Id: I6cbbdd09bc368b76fe82eb4800fd87b6fdcc0aa7
Tuleap: #896