Fix parsing of combined encrypted/signed messages

    In the case that a mime object was encrypted and signed the
    rendered mail showed an incorrect signature status because the
    signature was aready checked with the decryptverifyjob and the
    key object not available in writeOpaqueOrMultipartSignedData.
    (So only the fingerprint was used).

    In the case that writeOpaqueOrMultipartSignedData is called
    to show OpenPGP Encrypted and Signed data with a valid signature,
    the signature's key is now fetched from the OpenPGP backend using
    the fingerprint from the signature.
    The Key is then used as in the other cases to show the details
    (Name / Trustlevel etc.)

    This fixes the rendering of openpgp mails signed with the combined
    method which is used by mutt, gnus etc.

    BUG: 286035
(cherry picked from commit 3f8f80c1)