Started adding some sounds

I started adding some sounds since the game felt quite sad without anything playing.

I'm not too happy about the laser sound, firing the laser for too long hurts my ears after a bit, maybe I should decrease its volume.

Full changelog:

Added sounds:
- mine explosion sounds;
- player hit sound;
- laser firing sound;
- player explosion sound.

Set fixed timestep to false, should help with performance issues at the cost of stability (higher average fps but greater fps variation while playing);

Mines play sounds based on proximity to the player;

Small tweaks regarding asteroid spawning and starting speed.