nixos: environment.pathsToLink += some desktop dirs

Close #9622.
(adding common desktop locations and locations specified in
http://standards.freedesktop.org/menu-spec/1.1/)

nixos/modules/config/system-path.nix

···
         "/lib"
         "/man"
         "/sbin"
+        "/share/applications"
+        "/share/desktop-directories"
         "/share/doc"
         "/share/emacs"
+        "/share/icons"
         "/share/info"
         "/share/man"
+        "/share/menus"
+        "/share/mime"
         "/share/nano"
         "/share/org"
         "/share/terminfo"
+        "/share/themes"
         "/share/vim-plugins"
       ];