Adding hash function to SceneInterface

I've added a hash function with an enum that computes different kinds of hashes (for Object, Transform, child names, etc) for a given point in time.

This will allow better use of caching mechanisms when loading scenes as well as use of instances in engines such as Gaffer.

Hmm. Actually yeah, why not. Treating the transform as a property of the parent is a bit of a pain anyway, and it complicates that SceneElementProcessor::hashHIerarchy() procedure I described a while back.