Blender Git Loki

First step towards supporting non-blender-data as assets.

Idea is to avoid the need for asset engines to generate whole .blend files
with relevant datablocks in case they just need/want to provide files (images, videos, sounds...).

This commit introduces the concept of 'virtual' library, which in fact only exists to store
asset repository data. Datablocks from those virtual libs are then fully written in .blend file.

Much to do still, this is only preliminary/experimental work.