Implement a proper scene graph

[DeviateFish]

Any drawable should be able to have other drawables as children. This makes some assumptions that differ from the ones already existing: the first of which is that only things that extend from ModelDrawable have matrices for positioning.

However, nearly(?) everything extends from ModelDrawable, so it might make sense to refactor most of the functionality there into the base Drawable class.

The second part is allowing drawables to have children, and enabling propagation of the drawable's local transform to its children's world transforms (or something like that).

[DeviateFish]

Closed by #26