Changed the width and height datatype for Paddles.

Instead of std::size_t is now unsigned short, since that is more what it
actually is std::size_t has a different usage!
Added width and height functions to pong::Game.

They are inline functions which are merely return the values in the main
SDL_Surface*.
Added PaddleController::handleEvent.

It takes an event just like every other handleEvent function. It is pure
virtual.
Fixed up the MousePaddleController class.

- It is now compiled.
- It now uses #pragma once to prevent redefinitions.
- It takes a Game reference to const as a constructor argument as
opposed to using the singleton.
- Otherwise it works the same way!