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

[Jani Monoses]

> such a patch is not approproate at this point. It is nonintrusive
> however and I can make an uptodate patch if people thinks that it'd be

Actually it is intrusive if I think more about it :) . The only
non-intrusive part is supporting shutdown/reboot via HAL, but the PM
stuff required changes throughout xfce4-session.

Jani