Partial rewrite of Model3D for skeletal models.

- transfer the actual storage of a mesh from Model3D to Model3DMeshNode
  (which hides the variant nature of a mesh)

- finish to implement SkeletonBone, to handle and compute a bone matrix

- add a debug-skeleton import property to display the bones through
  a transparent hand (the bones are displayed at their rest positions)

- add 'rotate-bones' and 'roll-bones' import properties, used to adapt
  hand models that where designed in a Z-up space (such as with blender,
  for instance)