A recent change[1] caused the `NC_SCENE | ND_FRAME` combination to no
longer perform a full Outliner rebuild. However, both Alembic and USD
importers were using this combination. The end result was that the
outliner would not show any of the newly imported objects/collections
until a rebuild was forced some other way (like going into Edit mode on
an object).
Change to use a more appropriate notification that corresponds to the
concept of new IDs being added to the scene instead: `NC_ID | NA_ADDED`
Also, remove the notification being sent from the startjob callback when
creating a new collection. It's unneeded in this situation and it could
be problematic being called from the callback.
[1] b549260aa4
Pull Request: https://projects.blender.org/blender/blender/pulls/115883
2f0633f5fa