Make `internal` vs `public` consistent in the Gum tuotrials

[vchelaru]

User on Discord reports:

For the button, the class is declared as:
internal class AnimatedButton : Button { }

But the Slider is declared as
public class OptionsSlider : Slider { }

Is there a reason that one is made internal and the other public?

Review this and make it consistent with the rest of the tutorial.