power: Support automatic logout after a maximum period of idleness

This is very commonly requested in any situation with
shared resources: servers, labs, etc.

We either log out, or force the logout when the maximum idle time
has been detected.

The feature is controlled by the sleep-inactive-*-timeout and
sleep-inactive-*-type keys.

http://bugzilla.gnome.org/show_bug.cgi?id=597030