[FR] Example using stateful menu buttons

Brilliant series, by the way...

It would be awesome if you could describe the Kivy way of creating a reusable menu across screens in a screen manager which include state.

The example menu might have three buttons which would represent three different screens with a toggle (radio button) effect. As we then move on to the next screen, the second button's styling shows that it is selected, for example. In this way, we always know where we are (without having to add a title bar as seen in the 14th video).