Add support for powering off drives

This was removed in 3.4, now it's back.

http://people.freedesktop.org/~david/gnome-disks-power-off-drive-1.png
http://people.freedesktop.org/~david/gnome-disks-power-off-drive-2.png
http://people.freedesktop.org/~david/gnome-disks-power-off-drive-3.png

https://bugzilla.gnome.org/show_bug.cgi?id=675542



Signed-off-by: David Zeuthen <zeuthen@gmail.com>