Make it possible to restore a disk image

 http://people.freedesktop.org/~david/gdu2-disk-image-restore-1.png
 http://people.freedesktop.org/~david/gdu2-disk-image-restore-2.png
 http://people.freedesktop.org/~david/gdu2-disk-image-restore-3.png
 http://people.freedesktop.org/~david/gdu2-disk-image-restore-4.png
 http://people.freedesktop.org/~david/gdu2-disk-image-restore-5.png
 http://people.freedesktop.org/~david/gdu2-disk-image-restore-6.png

As evident in the first screenshot, the popup menu is slowly getting
out of hand. It would probably be worth-while to rework it so it looks
like e.g. the "Epiphany Mega Menu", see

 https://live.gnome.org/Design/Whiteboards/Menus



with one column for "Disk" and one for "Volume". Something to poke the
designers about.

Signed-off-by: David Zeuthen <davidz@redhat.com>