Improve locking the screen and shutting down, introduce suspend/hibernate

Hello,
I have written some code to enable suspending and hibernating the machine from rest break and daily limit windows. This has been a feature I have always missed in Workrave.
By the way I have also improved greatly support for locking the screen and shutting down.
All these things should now work correctly on most Linux installations currently in use.

Please review.
Greetings,
Mateusz Jończyk