/*
     * Ajoute un groupe au JTree
     */
    public void addGroupToJTree(Group group) {
         DefaultTreeModel dtm = (DefaultTreeModel) tree.getModel();
         DefaultMutableTreeNode root = (DefaultMutableTreeNode) dtm.getRoot();

         DefaultMutableTreeNode newGroupNode = new DefaultMutableTreeNode(group.getName());
         List<Ticket> tickets = group.getTickets();
         for (Ticket ticket : tickets) {
             DefaultMutableTreeNode newTicketNode = new DefaultMutableTreeNode(ticket.getTitle());
             newGroupNode.add(newTicketNode);
         }
         root.add(newGroupNode);
         dtm.reload();
    }