Adds a secret key to the key ring.
Index of the newly added key.
Use this method when you need to add new secret key to the key ring.
Key is added to key ring with all the corresponding subkeys, signatures and user-ids.
If such key is already present in the keyring only new information (signatures, subkeys, UserID) will be added.
Key is copied during adding so Key instance can be freed after the call to this method.