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.