adding support for hibernation to xfce4-sessions's exit dialog

[Jani Monoses]

> If you add a toggle button to "Logout settings", then that's no
> different than adding the buttons to the logout dialog: The user is
> still the one to figure out.
> 
> Nobody said it'd be easy to do it right. But it's way better if the
> developers spend a day or two figuring out, than having each user spend
> atleast one day to figure out.

User input is required anyway since the only way to tell if
hibernation/suspend works is by trying it out. The status reported by
the system is not 100% reliable.

So the choice is between opt-out or opt-in, but the latter needs to be
made easily discoverable.

And hibernation can work on desktops too, not only laptops, and it makes
sense for faster restart of the system.

Jani