Blender Git Loki

Asset Catalogs: explicit version number in catalog definition files

Declare the current format used for asset catalog definition files as
version 1, and write that to the files. Files without that version number
will be rejected.

This makes it much easier to move to different versions later, with the
opportunity to do versioning on file load.

The version is not associated with any version of Blender, but a separate
integer that's simply incremented when a non-backward-compatible change
happens.