Change approach for dconf, and this is working !

Now we use the special configuration of gnome to set dconf variables for all users (https://wiki.gnome.org/dconf/SystemAdministrators).
added keyfile, which contain configuration for a dconf key and launch a dconf update to root user and one user (if set). This permit to hot reload dconf and set the variables when user is logged.
Also add a lockpref/unlockpref to forbid variable modifications