Collect S/MIME Opaque as crypto data

* src/mimedataprovider.cpp (t2body): Start collecting crypto
data when encountering an smime attachment.

This also removes is and maybe opaque signed as this complicated
the code (actually needing some X509 parsing in GpgOL) and
is not needed imo. We can just try to verify it and verification
will fail if its not validly signed.