API: expose info about the signing key

it would be nice to have the "Message" object that the leap mail 0.4 will offer exposing the singnature/validation level too. In order to the knowledgeable MUAS to display the validation/trust level about the key, the mail interface needs not only to know if the signature is valid, but it needs to get some wrapper around a key object from the keymanager instance that the mail service will be using.

We need to think about what is the most convenient API for exposing info about the signing key in the keymanager.

(from redmine: created on 2014-11-13, closed on 2015-01-07, precedes #6363 (closed))