Feature: sign messages

I started a straight-forward implementation of signing messages. This is WIP and should _not_ be merged, yet. I’d like you to be aware, that I started working on this and would like to get some first comments on the direction I took here.

Beware, that this branch is going to be force-pushed to, if needed.

# ToDo

- [x] Show userid in password dialog
- [x] primary key is not auto-selected
- [x] fix cancel button in password dialog
- [ ] ~~basic refactoring (think about unambiguous variable names for signing and encryption)~~