refactor: add parameters to keysharecore.NewKeyshareCore instead of having to always invoke methods on the return value

* gabi.PublicKey, gabi.PrivateKey and related methods moved to gabikeys
* handle additional errors returned by gabi

Previously all wizard items not of type credential were forced to be last,
but it suffices to force only items having no credential type to be last
(regardless of their type), because only those cannot be taken into account
in the dependency expansion algorithm.

The revocation subcommands are now subcommands of irma issuer.