window: implement "reset from beginning" feature

In this MR I propose an implementation for #37 (closed) as a menu item. When the item is pressed, the application state is reset, similar to when you launch the application, starting at the first lap.

As an user of this application, I wanted to give it a try to the source code to learn more about gtk-rs too. I hope I am not skipping any design decission by offering an implementation 😅