Blender Git Loki

First 'working' version of asset reload.

Dead dummy simplistic test runs (nearly) OK here (only have a memleak from versionning code...),
but obviously will need much serious tests to validate everything.

Also, many things in code will need cleanup/refactor. Not to mention UI/UX.

But we do have asset version check & reload on file opening now! :D

Process summary:
I) Open .blend file exactly as in master, load assets just like any other linked data (add placeholders in case not found).
II) Check all asset engines and ask them whether each asset is OK, needs to be updated/reloaded, is missing...
III) User then can decide to actually reload the asset, or not.

Step II) happens in an async job (since engines using remote storage/database could take some time to answer),
step III) is a locking task (just like initial linking). This should allow user to not be blocked at all by that
asset update process.