Blender Git Loki

First step to add 'load_pre' behavior.

This will allow asset engines to:
* Actually ensure requested data is available (downloading it, generating it, etc. as needed).
* Change entries generated for user by actual data paths (allows e.g. to present
a set of different paths to load as a single 'asset' to user).

As usual, had to change existing filebrowser code... Seems to be working fine
from quick tests (i.e. regular filebrowser not being broken).