[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