desktop-entry-spec: Add an optional Keywords key

[Florian Müllner]

With search gaining traction as being the preferred way to locate
applications (not only in GNOME), the existing .desktop file fields
meant for browsing often produce insufficient results.
For that reason, gnome-control-center introduced a custom
X-GNOME-Keywords field (see [0] for an example), but as the key would be
useful for other processes searching through .desktop files, I'd like to
propose it for standardization.

Regards,
Florian

[0]
http://git.gnome.org/browse/gnome-control-center/tree/panels/mouse/gnome-mouse-panel.desktop.in.in