Blender Git Loki

Asset API: add Views.

This commit add new 'View' forth layer, below history one (Revision).

It also adds a real UUID for the asset repository (as usual, asset
engines not needing it can just set it to zeros), will make Amber's life
(and all multi-repositories engines) easier.

Also fix one or two issues found on the road.

Code builds and links datablocks from libraries in the 'old way'. Real
test will come when re-enabling Amber and adapting it to new API though.