✨ feat(MainWindow.ui): add a label and a spinbox to display the number of graphs selected
The graph_type_changed method is added to handle changes in the graph type selection. It updates the number of graphs selected and displays it in the newly added spinbox. The label is also added to display the text "Nombre de graphes :".
feat(MainWindow.py): add 'has_changed' and 'old_discretisation_value' variables to track changes in the discretisation value selector and avoid unnecessary computations
fix(MainWindow.py): set status message when rendering graphs to inform the user of the process
