Right click on empty space to add a state.
Right click on a state to delete it.

* yavgui/src/automaton_instance.cc: Cosmetic.
* yavgui/src/g_painter.cc: Do not delete the scene when refreshing;
update it instead.
* yavgui/src/g_painter.hxx,
* yavgui/src/g_scene.cc,
* yavgui/src/g_scene.hh,
* yavgui/src/g_state.cc,
* yavgui/src/g_state.hh,
* yavgui/src/g_vcsn_handler.cc,
* yavgui/src/g_vcsn_handler.hh,
* yavgui/src/g_vcsn_handler.hxx,
* yavgui/src/g_transition.cc: Add necessary signal/slots/connections.
* yavgui/src/vcsn_automaton.cc,
* yavgui/src/g_view.cc,
* yavgui/src/g_view.hh: Remove obsolete signal/slots/connections.