Remove all OpenGL dependencies from Core project

[japsuu]

The Core project should not be aware of the active graphics-API implementation. This can be achieved by removing OpenGL related classes from the KorpiEngine.Core project and placing them in their own project (KorpiEngine.OpenGL?).

This way the user can reference the desired Graphics API project from their own project, and when they call Application.Run to initialize the engine, they can pass in a reference to the desired "graphics API instance" implementation (IGraphicsAPI?).