gnome-panel-desktop: Fix the Categories key
gnome-panel-desktop: Fix the Categories key

gnome-panel is not an utility, it is part of the core.  Hence,
the Categories reflects panel is part of System.

https://bugzilla.gnome.org/show_bug.cgi?id=640690

Signed-off-by: Germán Póo-Caamaño <679831b636f7f6776b5d7665ebec4c0074d994a9@gnome.org>

GNOME/gnome-panel,GNOME/gnome-panel