To make more use of TOFU in the UI we need support for TOFU_STATS in gpgme's key
data type / keylisting etc.

This information could be included in listed keys when
GPGME_KEYLIST_MODE_VALIDATE is provided for OpenPGP keys or if this is not
expensive in every keylisting?

We also need tofu information in the decrypt / verify results i think, even if a
UI could check this through the key.