Add a relative positioning system.

[RubyNova]

We need a way to simplify calculating the positions of objects relative to other key points in the window. Specifically things such as the world space margins and probably other objects - essentially kind of like a "local space". It would mean yet another plane of space but it would resolve LLAPI positioning concerns raised by @tannergooding and it is something I was considering adding for the HLAPI anyway, so how we go about this is open for debate.