Support for textures

[Ghostkeeper]

While this application serves my personal need in its current state, I'd like to expand its functionality to what other people might need. One of the pillars of this is to be feature-complete for 3MF. While there are many features in 3MF, one of the most prominent ones is the ability to store textures.

The OBJ format that ConvertTo3mf currently reads has support for textures as well. To implement this feature, we need to implement reading textures from OBJ, storing them in our intermediary data structure, and then writing them to the 3MF file.