Implement user input via Unity's new input system

[achimmihca]

Problem or question to be solved

At the moment only keyboard and mouse input is implemented in UltraStar Play.

Suggested solution/s

In Unity 2020.1 the new input system for Unity will be available in a stable version.

User input should be implemented using the new input system and documented.
I prepared a wiki page for this: https://github.com/UltraStar-Deluxe/Play/wiki/Handling-User-Input.