Blender Git Commit Log

LibOverride: add checks to RNA collections editing utils.

Collections of liboverride data only support insertion of new items (and
deleting or moving those around). No operation on existing items from linked
data is allowed.

Reflect that in RNA functions used to edit py-defined RNA collections.

Note that this is not ideal currently, it does the job but feedback to
user is rather poor. Ideally add-ons should implement higher-level
checks for override cases in their operators.