Drafted shaders feature

docs/user/features/index.rst

docs/user/features/properties/index.rst

docs/user/features/properties/material.rst

docs/user/features/properties/niproperty.rst

docs/user/features/shader/index.rst

docs/user/features/workflow/index.rst

io_scene_nif/properties/shader.py

io_scene_nif/ui/shader.py