Route and Navigation

At the moment the application only contains one screen.
To be able to display different content (e.g. all completed tasks) we can use routes and navigation system.
The current documentation linked to the following article https://medium.com/flutter/learning-flutters-new-navigation-and-routing-system-7c9068155ade as the routing system has been updated recently (see https://flutter.dev/docs/development/ui/navigation)

- [x] Create a new route which display all the completed tasks
- [x] Create a home route which allow the user to go back to the main page