PowerUp handles checking if it is obsolet by itself.
Move using power up into PowerUp.
Extract method for adding bubbles.
Move checking if torpedo can be removed into Torpedo.
Fix PowerUp.
Move check for enemy's obsoletness into Enemy.
Remove logging accidently left in.
Unify handling of torpedos, enemies and power-ups.
Handle collisions between torpedos and enemies via external system.
Use collision system for player/enemy collisions.
Handle player/power-up collisions via separate system.
Move enemy creation into separate system.
Move creation of random bubbles into separate system.
Move game over condition into world.