(In ) Model code: Add support for importing smoothing groups to all model loaders.
The loaders of all model file formats have been augmented to import smoothing groups, whenever such information is available in the imported file.
The imported smoothing groups data is stored in our appropriately extended `cmdl` files, and the `CafuModelT` class has been extended accordingly.
Still missing is
- the actual ''use'' of the newly available information (at this time, we continue to compute tangent-space as before),
- thorough testing for each of the supported file formats.