* Added Q/Esc keys listeners to quit the game, showing the overlay with buttons to confirm or continue
* Refactoring in Board for better definition of overlay and buttons, including refactoring css style classes
* When restoring a stored session, check if 2048 tile is already found to avoid the won message if others 2048 tiles are found.
* Added Enter/Space keys listeners to buttons in overlay, which are focused when the overlay is shown.