Mwoo dev

Merge to master so
- Cannon can rotate, fire, have a cooldown, change element mats depending on projectile element, and handle Projectile and Explosion data structures;
- Projectiles can travel distance, explode on contact, and change element mats;
- Explosion can increase in radius over its duration and change element mats;

- PowerUpManager can spawn PowerUps according to a spawn timer and spawn chance, activate and deactivate PowerUps, and handle PowerUp data structures including inactive PowerUps, active PowerUps, and effective PowerUps.  PowerUpManager also moves PowerUps so that the player can see he/she has some active;
- PowerUp moves (according to gameplay and once triggered), retains only the necessary information about itself (duration of effect, movement speed, level, and type), and can change element mats.