Change score bonification logic. Add ShieldShip class with score
bonification for time.
Now the size of world is the same in all devices. Media contructor
receive a screen world size. the stage set a viewport equal to screen
world size in media.
Add sound attribute to Entity class
Add methods to load sounds and music.
fix error on Background. add sound attribute to Background
Add sounds of game.
Add music to World.
Remove temporarily bSpeed button of GameUI
Fix: showed no score
Load sound for final explosion
add init generation time for asteroids.
Change volume of game music.
Fix: reset score on try again.
Change text of Message Label