add controller logic to do signing

all the logic to do the actual signing within the controller. Shares a little
code with the decryption logic (unlock private key for signing).
add sign button to encrypt frame

This hooks up the logic to the UI, so it can be used. This is also not the
final version, because in the end there would be 3 states:

* sign message only
* encrypt message only
* sign and encrypt message

So the UI needs to be changed to represent all those states.