Set NoDisplay=true for Disk Image writer and Disk Image Mounter apps

This prevents these applications from being displayed in the menu but
still allows them to be visible in Files, as intended. See
https://bugzilla.gnome.org/show_bug.cgi?id=702681

 for the bigger
picture. Cosimo, who did the underlying change, obtained release-team
approval for this late change.

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