Blender Git Loki

Fix T44453: Exporting Keying Sets referencing node tree properties generates invalid Python code/paths

The problem is that node trees (such as the Material, Lamp, and Compositor node trees)
are stored as "nested node trees" on the affected datablocks. They are particularly
troublesome to deal with, as the are not easily identified, and also cannot be easily
mapped back to the ID's which actually own them. As a result, the usual automated
methods do not work when dealing with these!