AMF Import

This is based on the SVG Import (mainly as it's also using libxml2 for parsing), so it should be only considered after #834 is merged.

The import supports reading multiple objects which are currently joined into a single object (that might be changed later when #350 is available).

No support for curved triangles.

ToDo:
- [ ] verify how to handle multi-volume objects
- [ ] add support for reading zipped files