Blender Git Loki

Keep assets local to the file, don't copy them

Previously my idea was that when creating an asset for the user
repository, we'd always create a deep copy of the data-block for storage
in the repository.
However for the initial design, we decided to not let Blender
automatically save things into a repository. The user has to mark a
data-block as asset and save the file, so the asset is stored as part of
that file. By adding a path to the Preferences, all .blend files in that
path become part of a repository and users can then access their assets
in different projects through the Asset Browser (not done yet).

Because of this, the operator is now called "Make Asset" again, not
"Create Asset". We don't create a new asset, we just add asset data to
an existing data-block.