Blender Git Loki

Asset Catalogs: write catalogs to disk when saving the blend file

The Asset Catalog Definition File is now saved whenever the blend file
is saved. The location of the CDF depends on where the blend file is
saved, and whether previously a CDF was already loaded, according to the
following rules. The first matching rule wins:

1. Already loaded a CDF from disk? -> Always write to that file.
2. The directory containing the blend file has a
`blender_assets.cats.txt` file? -> Merge with & write to that file.
3. The directory containing the blend file is part of an asset library,
as per the user's preferences? -> Merge with & write to
`${ASSET_LIBRARY_ROOT}/blender_assets.cats.txt`
4. Create a new file `blender_assets.cats.txt` next to the blend file.