Initially adding rendering code to project

Rendering code has arrived. It's a bit handicapped at the moment as we need to feed it some data so currently it only renders the outlines of objects in the SpriteKit level designer.

I couldn't figure out how to provide any custom data using that editor - neither custom (subclassed) node types nor userData for existing nodes seem to be exposed for any of the nodes that can be added to a scene through the level editor. Without this, we can't customize the rendering of any node (color, thickness, other sketch material parameters.)