Blender Git Loki

Initial basic support for assets definition in .blend file itself, and RNA api access.

This still *very* basic, but should be usable as proof of concept for
Cosmos pipeline tests...

Note that the question of where to put all asset metadata remains open,
we could extend AssetUUID struct, and/or add support of IDProps to it,
or just use some special-named IDProp in IDs themselves (like the
_RNA_UI thing for custo; props UI settings...).