combo box events

[Martin Spurek <martin spurek fei vsb cz>]

Hello all. I was trying to find out how to grab event on opening combo box, but
i did not figure out. In combo box group of classes, there is just
Glib::SignalProxy0<void> signal_changed() and in hirerarchy above just in
Widget some signals like Glib::SignalProxy1< bool, GdkEventButton* >  
signal_button_press_event() or Glib::SignalProxy1< bool, DirectionType >
signal_focus (), but i am not sure how to make them do what i need. There is
not much docummentation. Thanks for any advise.
martin