add sign icon from crystal icons
add inline sign dialog

This is basically a copy of the encryption dialog and will be added to the
inline encryption frame in order to sign the text in the editor.
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.
Merge pull request #152 from johnyb/feature/sign_messages

Feature: sign cleartext messages