Blender Git Loki

Python API: allow external render engines to use Eevee for LookDev shading

This is enabled with bl_use_eevee_viewport = True. It allows external engines
to generate an Cycles/Eevee shader node setup to emulate their materials in
the realtime viewport, or to leave it to users to do manually.

Removed bl_use_exclude_layers and bl_use_shading_nodes that did nothing
anymore. This should not break API compatibility, any scripts setting those
should continue to work the same as before.

Also adds descriptions for some RenderEngine settings.