Added a default initialized constructor for GameState. Couldn't hurt!
Added a new SinglePlayerGameState which is given control when the user clicks the "Singleplayer" button in the main menu.
Added \file descriptions to src/EventSignals.cpp and src/EventSignals.h.
Added the start of the Paddle class, it isn't usable, but it's a start.
Removed the include of "Paddle.h" in src/SinglePlayerGameState.h since it really doesn't need to be included.

If anything it's increasing build times.
Added some constructors and assignment operators to the Paddle class to: 1) prevent moving and copying, 2) allow trivial construction and destruction, and 3) allow a derived class's destructor to be called.

In english:
 - Added a default constructor.
 - Added a virtual default constructor.
 - Added a deleted copy constructor.
 - Added a deleted move constructor.
 - Added a deleted copy assignment operator.
 - Added a deleted move assignment operator.
Documented pong::EventSignals in src/EventSignals.h.
Added the ABC for controlling a paddle and included in Paddle.h so it is compiled.
Added newlines to the end of src/Paddle.h and src/Paddle.cpp, I had forgotten to add them before.