Add a looping background music

[Indy2222]

Make it very simple (improvements will be tracked by different issues). It can be a single song looping indefinitely in menu and game.

[Indy2222]

We can reuse music for the original Digital Extinction (from 2010) made by DJ Chemi.

https://drive.google.com/drive/folders/1yTlklHNCeqoe7ZGNsh72aQdQ53AJnXbi