Fix .desktop file categories so that nautilus will show up in gnome-software

gnome-software uses the desktop file categories to sort the applications
it shows.

This commit fixes the categories according to the freedesktop spec,
so that nautilus will appear in the correct category.

For more information:
http://standards.freedesktop.org/menu-spec/latest/apa.html
http://standards.freedesktop.org/menu-spec/latest/apas02.html