Give a player three lives

[michielpapenhove]

A player should have three 3 lives when the game starts. If they are hit by an alien bullet or hit by an alien, the amount should decrease by 1. If the amount reaches 0, the game is over.

This might be a splittable story:

• Add a property on the player for the livesLeft
• Reduce the number of lives when hit by an alien bullet (requires Have aliens fire bullets too #28)
• Add an overlay/HUD that contains the number of lives so it's visible for the player
• End the game when the lives reach 0