Removed the unnecessaries. Files which have been created but haven't been used.

Also removed any references to those files: `#include`s, CMakeLists.txt and some code.
Changed the GameState's public, virtual render and update functions to regular functions which call a private counterpart which needs to be overrided by any derived class.

These changes are to prepare for the event system overhaul.
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