Update 1.5.8 - FPS - PVE Balance - Background

Update 1.5.8 makes the game more casual and player friendly:
- Enemy strength and spawn timer growth changed to non-linear
- The game speed balanced and now stable
- Game background colors tuned to not disturb battlefield view

Huge thanks to StormShadow277 for the his feedback

in plans: Re-Charging shield. Multiplayer.