Adjust timers in the storyboard

Adjust the time each frame will be updated in the storyboard. Now, each
frame will last 4.5 seconds in the game screen.
Refactor: remove not used files from the project

- Remove some assets that are not used in the game. Besides, change some
files that reference such assets.
Refactor: remove commented code

Remove not used code from the project.
Refactor: improve code style

- Replace all tabs by spaces in source code;
- Put at most 100 columns per line.
Bugfix: Background appears only after loading

Fix a bug in the Level2Preloader state, in which the background only
appears after the progress bar is complete. The right behavior is the
background appears before loading.