Add convenience function to get key from sig

* lang/cpp/src/verificationresult.cpp (Signature::key(bool, bool)):
New. Can be used to search / update the key associcated with this
signature.

By using update a caller can ensure that an incomplete key
obtainable through the new key() function is fully loaded.
With search the key can be looked up in the internal keyring.

As the results are cached this can be done in the crypto thread
and the result then better used in the UI thread.