desktop-app-info: Add support for X-GNOME-Keywords

With search gaining traction as being the preferred way to locate
applications, the existing .desktop file fields meant for browsing
often produce insufficient results.
gnome-control-center introduced a custom X-GNOME-Keywords field for
that purpose, which we plan to support in gnome-shell as well.

https://bugzilla.gnome.org/show_bug.cgi?id=661763