Proper composite support in GenericImporter

[mwestphal]

Is your feature request related to a problem? Please describe.
With the addition of a MetaImporter in #1609, it is now possible to add proper support for composite data in the GenericImporter

Describe the solution you'd like
When opening a composite data, the generic importer should, instead of merging geometries together, generate one actor by composite block.

Appropriate image and point sprites accessor should also be updated.

The vtkF3DRendererWithColoring should then use these accessor to handle actors visibilities correctly when showing volumes and point sprites.