When clicked on the "Open" Button at the Main Page, add the possibility to choose one or more .txt and .g6 graph files.

When it happens, the Project Window opens and fill the checkbox with the selected graphs and set the page name with "Visualization Mode".

• Add possibility to choose .g6 and .txt at Main Window
• Change controller behavior when selected anything different than .json
• Fill the project_information_store with the graphs and the name of "Visualization Mode"

To change the controller behavior, update the controller.py file at function show_open_project_window, line 59