[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

[gui][lib] ChartModel: Fix bug: after notify changes the model status didn't return to the unmodified state
- This caused unecessary computations
- This prevented optimizations

[gui] chart_misc_widgets: Display previously selected nodes on the model after a switch to another model

New nodes created manually are named with are named taking into account the names of the nodes already present (avoids accidental overwriting in node_dict