Fix categories in .desktop file, bug 707373

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 Cheese 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