Major refactoring

Created two new classes: ChessPiece.cs and King.cs, which inherits
ChessPiece.cs. Created a new class "Engine.cs" and moved all the code
from KingSurvival.cs to it. I'm drawing inspiration from Joro's
AcademyPopcorn game. Project is buildable (amazingly...). A personal
request to all commiters: please include more detailed descriptions of
your commits.