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

[Jani Monoses]

> 
> Yes, with the previously spawned shutdown helper (required to test
> whether sudo needs a password).
> 
> BTW: IIRC, there's already such a patch available in Xubuntu.

Indeed there is, but not using the sudo helper. It talks directly to HAL
and could be used on any other modern linux distro (and FreeBSD once HAL
is fully ported). As xfce is in feature freeze I got the impression that
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
good to have in 4.4.

Jani