Gameplay loop

Gameplay loop is pretty much done, I've added :

- Score update on kill

- Enemy respawn on full clear

- UFO every 15shots

- Player losing lives and game over detection

I have temporarily disabled bullets colliding together due to a bug

Next steps :

- Proper death and game over feedback

- Sounds

- Better visuals (colors, particles, UI)

- Fix bullet collision

- Tweak values for better game balance