data in signal association. (user_data on GTK+)

[John Holmes <merli renato gmail com>]

   In GTK+ we have a gpointer (called user_data) associeted with signal handlers that are specificated in the signal association. So, its a per-association value. How to have something like this in GTKMM (libsig++). 
   The following code don´t work:

   Gtk::MenuItem * tmp_item;
   refXml->get_widget("mn_add_dvd", tmp_item);

   (tmp_item)->signal_activate().connect( sigc::bind(  sigc::slot (*this, app::cb_menu_click, "test")));