Added a small structure which holds every signal related to events in a central place.

Also a universal function was added to the same file which handles events and calls the corrosponding signal, given an instance.
Added a EventSignals member to the GameState and set it up so that events are handled every update. Silently.

Also removed event checking in the MenuGameState. Making MenuGameState::update() a no-op.
Adapted the Button class to use the new event system.
Adapted the MenuGameState class to work with, and embrace (what, lol =P), the new event system.

The buttons are in slightly different places, and the multiplayer doesn't quit the game but otherwise, I think it's basically the same.
Added a "Help" button, my plan is to increase default ball speed when it is clicked. Otherwise the user wouldn't know! Trolalala