Blender Git Commit Log

Refactor how we store ID and asset data references in file lists

* Remove ID-map and ID session-UUID code, we can now reference the ID directly.
* Store imported asset metadata separately.
* Because of that, `FILE_TYPE_ASSET_EXTERNAL` could be removed.
* Get asset metadata of the current files via the ID directly.
* Move local data references into a nested struct, to group them together and
allow a clear comment on how they must be updated.
* Improve comments and note how the local data needs to be updated.