Multi-branch merge - see description

Multi-branch merge to rebase current workflow.

- Adds NormalMap import/export
- Update normalmap test
- Initial Bmesh API migration
- Adds Specular import/export
- Adds Specular tests
- Moved from tess face -> polygons
- Adds helper function to get_used_textslot
- Adds helper function to get_uvlayers
- Updates to use polygon loops
- Fix uv layer detection
- Update vertex color import/export to BMesh API
- Updated inter-mesh seam smoothing to BMesh API
- Duplicate textures detection throws error.
- Added unit test for get_used_slot