[omemo] Pass trust level to gajim

_message_received now passes the trust level of the key with which the message was signed to Gajim, to enable showing this in the UI.

If merge request 383 for gajim is accepted, then this will result in the UI indeed making that information accessible to the user. In other versions of gajim this change shouln't lead to any changes noticeable to the user.

I assume that the database should hold exactly one entry in identities such that the public key has the fingerprint from which we just got a message, so I assert this before using the first entry.