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

tangent-opensource/coreBlackbird,tangent-opensource/coreBlackbird,tangent-opensource/coreBlackbird