[gui] Improve get_simple_node_names and the calculation of signal_transition_dict (useless if transitions have not changed)

- Delete redondant code
- Add doc

=> deletion of useless get_model accessor

- Proper way to access to the model via XML parser
- Now the parser sets the status of the model to not modified just after the load of the file
- Fix typos

[gui] Charter/ChartModel: Fix modified model notification during the close; partially reverts 89fed44f

[gui] Charter: Rewrite on_display_states: show influencing places of conditions from the GUI; display missing nodes

[gui] CharControler;EditMVC: Refresh ChartInfo widget when a new model is set/switch, or following a deletion
- => just select the root/parent node by default

- Moreover in all cases of deletion, current_node must be reset