Change 1/3 - Move Powerup from extending Mob

Previously, the Powerup class extended from the Mob class just to inherit the move method in Mob. However, it also inherited the destructible interface even though the Powerup class is not a destructible entity, as well as the blockedX and blockedY attributes that Powerup never uses.

Powerup now extends Entity directly instead of Mob.  There is now a move() method inside of the Powerup class that performs the same actions it did when using move from Mob, without the unneeded changes to the blockedX and blockedY attributes.

There now exists an interface called Moveable, which was created to make all entity objects implement this interface if that object needs to move in game. This is because not all objects that extent the Entity class use the move() method inherited from Entity, including Powerup.  Therefore, the move() method was removed from the Entity class.  All classes that implement move and extends Entity now implement the Moveable interface, including PowerUp.