#59 Z depth level supporting composite

This will have to store a list of renderables like current Composite node. However, z index should be stored somewhere near related nodes. One possible idea would be to have simple ZIndex {Renderable source, Real index} node and store list of them in ZComposite.

And, as extension, pseudo-3D composite:

!Pseudo3D
  scale: true
  camera: (x, y, z)
  content:
    - !Pseudo3DElement
      source: ...
      position: (x, y, z)
    ...

~

!List/Renderable
  - !Transform
    source: ...
    transform: !Scale
      source: !Translate
        ...

Depth-of-field blur can also be applied in similar manner.

Z depth implementation via sorting:

!Sort
  source: !List/ZDepthElement
    -
      source: ...
      z_depth: xxx
  sort_node: "ZDepthLess"