Allow use of lights from parts

[KeithCarew]

Currently lights can be defined in parts and sub assemblies but they become un-available in other assemblies or parent assemblies.  Ideally one owuld define a light fixture with a light source and then place it into a parent sub-assembly to create a rendering.  The parent assembly would be able to use the light definition from its sub assemblies and parts.

This is not current functionality in spite of being obvious.