Represents a "shard" of a person from a single source (a single Backend), such as
an XMPP contact from Telepathy or a vCard contact from evolution-data-server.
All the personas belonging to one physical person are aggregated to form a single
Individual representing that person.
Properties of a persona are provided by implementing "details" interfaces, such as
NameDetails or EmailDetails. They must be accessed through these interfaces.
Different backends' subclasses of Persona may implement different sets of
interfaces. The set of interfaces implemented by a given persona is guaranteed not to change over the lifetime of that persona.