shell-app: Make use of Keywords in search

.desktop files have been designed for browsing, so the existing
fields often produce insufficient results when used for search.
gnome-control-center used X-GNOME-Keywords for that purpose, which
has now been standardized as Keywords. It makes sense for us to
support it in gnome-shell as well (and encourage its use outside
of settings panels).

https://bugzilla.gnome.org/show_bug.cgi?id=609702