Add the use viewport to ensure stable behaviour on different devices

[TheMangelo]

You may want to have a stable playing environment regardless of screen size.
Well, let's talk about how we can achieve this. We can do this by creating a
Viewport instance and a camera instance.

First, what is a viewport?
In LibGDX terms, it is a way to manage a camera, mapping screen coordinates
to the games world's coordinates. LibGDX comes with the following Viewport
implementations, but you can also create your own: