protege-distribution: extend desktop file for grouping

Specifying 'categories' allows Protege to be consistently grouped
inside application launcher submenus.

pkgs/development/web/protege-distribution/default.nix

···
       desktopName = "Protege Desktop";
       icon = "protege";
       comment = "OWL2 ontology editor";
+      categories = "Development";
       exec = "run-protege";
     })
   ];