When loading old blend files, versioned Actions can end up having a slot with an
'UNSPECIFIED' `target_id_type`. Assigning the slot to an ID will then set the
slot's `target_id_type` to match the type of the ID, but there was previously no
way to directly set it via Python if needed.
This PR changes the writability of `target_id_type` to match the extent of its
mutability when assigning a slot to an IDs. Which is to say, it can be set if
it's 'UNSPECIFIED', but not otherwise.
RNA doesn't have a good way to represent this, so we accomplish this with a
custom set function that simply ignores the write if the slot's `target_id_type`
isn't 'UNSPECIFIED'. This isn't ideal, but is the least-bad solution at the
moment.
Pull Request: https://projects.blender.org/blender/blender/pulls/133883
Nathan Vegdahl
b5005cd99c