added wall entity
hide bullets on overlap with wall
store entities in hashmap
set actor random position on init
set wall random position on init
random wall movement in new Entitity.update() method
removed unnecessary method
make use of new update method in Actor class
added constructor comments
moved entities to separate package
freeze all entities on game finish
implemented specific actors and consistent Entity.paint method
impemented BulletEmitter as ArmedActor
implemented entitity IDs
simplified actor movement
implemented MiniGameObserver and EntityManager
moved BULLET_SPEED
check collisions in MyMiniGameObserver
improved collision check