Blender Git Loki

Simplified cache item path description, based on plain ID blocks with
subtype/index.

This omits possible instancing and recursion by dupli groups, but since
the data of instances is always the same in Blender at this point, there
is little need to take them into account for caching.

At some point in the future it may become desirable to store full dupli
hierarchies in the cache, but it doesn't make sense to try to design a
fully-fledged path descriptor for this hypothetical case now.