Add keywords, as recommended here:

 http://standards.freedesktop.org/desktop-entry-spec/latest/ar01s05.html,
 http://bugs.debian.org/693918, and
 https://wiki.gnome.org/Initiatives/GnomeGoals/DesktopFileKeywords
Keywords value MUST end with a semicolon
Merge pull request #437 from levitte/gnome-desktop-add-keywords

Add keywords to darktable.desktop