Add interface file for Projectile

We need to take projectile damage and collision rectangle to determine
when and how many lives we need to substract from the bat lives.
Rename Projectile interface to GenericProjectile

To avoid confusiion
Implement getProjectiles() method for PolarStar

Return existing projectiles flied from the PolarStar.
Make actions on bat colliding with projectiles

We return collision rectangle from the bat and from the Polar Star
projectiles. In the Game::update method we define what we need to do
when this collision happens.

Also, add alive_ field to projectile to determine when the projectile
need to disappear if it collides with an enemy.
Implement bat takeDamage method

To take bat HP on collision with projectile we need a method to make a
damage to it.