Hint when no shortcut defined

Added a hint when no shortcut was defined (instead of displaying "Press (null) to ...").

![Bildschirmfoto 2013-04-09 um 10 03 33](https://f.cloud.github.com/assets/637549/355732/058dade4-a0ec-11e2-9477-d22942b1f8d7.png)