Improved Gameplay and Changed Balloon Sprite

- collision detection improved ( balloons[i].y + 90 );
- dart speed changed ( 2500 to 1300 );
- toolbar ( score bar ) position changed to top and now is transparent;
gameplay and graphics improvements #2

- Changed Background image on main window;
- Added cooldown to Dart before can shoot again;
- Deleted "Balloon.png" from resources ( not used anymore );