==Proposal for CSG reoranisation==

CSG is adding a lot of new code for generating certain primitives, for example
CSGBox and CSGSphere. Most of those primitives (and more) already exist in the
PrimitiveMesh class, which is used by MeshInstance and CSGMesh.

My proposal thereby is:

* Make CSGMesh the top CSG class like so:
Object
|-Node
  |-Spatial
    |-VirtualInstance
      |-CSGMesh

* CSGMesh gets the following properties:
  => All properties of CSGShape
  -> Property of type "Mesh" for choosing a mesh or PrimitiveMesh
  -> Property of type "Mesh" that contains the merged mesh of itself
     and all its children

* The CSGBox, CSGSphere, CSGCombiner, etc. classes are merely added as syntactic
  sugar. They are all direct children of CSGMesh.

CSGBox inherits from CSGMesh and has its mesh set statically to CubeMesh.
CSGSphere inherits from CSGMesh and has its mesh set statically to SphereMesh.
...
CSGCombiner inherits from CSGMesh and has its mesh set statically to null.

That way, the Mesh subclass PrimitiveMesh is reused for all shapes available in
CSG, which increases code reuse and makes the structure more consistent.