The Paddle class now has copy semantics!

They are explicitly-defaulted, but they work fine, since the paddle only
has two unsigned shorts and a math::vector.
Removed scoped_init.hpp
Made the PaddleController an EventHandler.

The PaddleController already had a handleEvent function so it was a
matter of including EventHandler.h and inheriting from the class.
Fixed the Paddle render function.

The width and height of the rectangle to fill wasn't set before, now
it is being set to Paddle::width_ and Paddle::height_ respectively.
Removed the include of render_text.h in Button.cpp

It is already being included from Button.h!
Added defaulted functions to the Ball class.

That would be a default constructor, copy constructor and copy
assignment operator all explicitly-defaulted. This is to tell my future
self that the compiler generated copy semantics and constructors are
valid and usable!
Modernized the SinglePlayerGameState.

The "SinglePlayer" button has also been enabled in the menu. Currently
though, there is a bug where after pressing a key you will return to the
menu but the mouse will not be freed or shown, that is because the
MenuGameState is not constructed again. It is time to add a couple of
functions along the lines of onFocus and onUnfocus.