Making Core abstract and creating Game subclass.

Passing Core size information and scene creation to Game class.
Game should use the Core and configure it as required.
Making IScene abstract and creating a subclass for it.
Merge: Making Core and IScene abstract.

Core and IScene shouldn't be instantiated, they should be extended.
Core gives the basic functionality while IScene encapsulate stages for the game.
New standard for includes with normal slashes.

Changing all backslashes to normal slashes.