Add support for restoring XZ compressed disk images

Also include [1] MIME types for disk images, e.g. introduce

 application/x-raw-disk-image
 application/x-raw-disk-image-xz-compressed

Also add a new desktop file for a "Disk Image Writer" application that
associates these mime-types to the command

 gnome-disks --restore-disk-image %U

to allow the end user to very easily copy a disk image to a device
from either his desktop shell or web browser. This feature is useful
for OS vendors distributing e.g. "rescue images".

[1] : Eventually these mimetypes should be part of the standard
shared-mime-info distribution from freedesktop.org, see

 https://bugs.freedesktop.org/show_bug.cgi?id=64845



for these. Until that happens, we'll just ship them as part of
gnome-disk-utility.

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