Add EnginioQmlClient::isAuthenticated read-only property

The property significantly simplify EnginioModel usage when a data is
protected and a query had to be executed after authentication.
Decouple EnginioIdentity from EnginioClient.

The change allows use to use one EnginioIdentity with multiple
EnginioClient instances and it solves ownership problem.
Fix issue of having multiple Identities before clientInitialized signal.

clientInitialized signal should trigger authentication check only on
the last identity.