Add shortcut navigation

Would be nice to have keyboard navigation, I always used it for gmail. Using the VIM-friendly navigation keys doesnt sound like a bad idea:
- j left
- k right
- u up
- l down
- n next message
- p previous message
- r reply
- f forward

On Qt, this tutorial might help, using Qsequence:
http://doc.qt.io/qt-5/qshortcut.html

Regards.
