Blender Git Loki

update Collada Importer: reworked export and import of Materials

- added import of transparency and emission into principled BSDF Shader
- added support for importing all default collada material parameters

* diffuse
* emission
* index_of_refraction
* shininess (mapped to BSDF Roughness)
* reflectivity (mapped to BSDF Metallic)
* transparency + transparent mapped to BSDF Alpha)
* ambient (creates unconnected texture node)
* specular (creates unconnected texture node)
* reflective(creates unconnected texture node)

- added support for exporting collada material parameters:

* diffuse
* emission
* index_of_refraction
* shininess (mapped to BSDF Roughness)
* reflectivity (mapped to BSDF Metallic)
* transparency + transparent mapped to BSDF Alpha)

- prepared support for exporting the following parameters
but currently commented out:

* ambient (creates unconnected texture node)
* specular (creates unconnected texture node)
* reflective(creates unconnected texture node)

Problem: For now we only allow export of principled BSDF based
materials. I am not sure from where to get ambient, specular
and reflective as those values are not included in the
principled BSDF Shader (wip).