`Action.slots.new()` in the Python API previously took either an ID or nothing
as a parameter. In the former case it would create a slot with the appropriate
`id_root` and name for that ID. In the latter case it would create a default
slot with an unspecified `id_root` and default name.
This had several issues:
1. You couldn't create a slot with a specific `id_root` without already having
an ID of that type. In theory this isn't a problem, but in practice in larger
scripts/addons you don't necessarily have such an ID on hand at the call
site.
2. You couldn't directly create a slot with a desired name without an existing
ID with that name. This isn't so important, since you can always just set the
name afterwards. But it's a bit annoying.
3. Most other `new()` APIs in Blender *require* you to specify the name of the
item being created. So calling this with no parameters was violating that
norm.
4. Ideally, we want to eliminate unspecified `id_root`s, since they cause other
weirdness in the API such as slot identifiers changing upon slot assignment.
To resolve these issues, and just generally to make the API more
straightforward, this PR changes `slots.new()` to take two required parameters:
an ID type and a name. For example:
`slots.new(id_type='CAMERA', name="My Camera Data Slot")`.
This fully specifies everything needed for the slot identifier upon creation,
and doesn't require any outside data items to create a slot with the desired
type and name.
In the future if we decide we still want a `for_id`-style slot creation API, we
can reintroduce it as a separate function.
Ref: #130892
Pull Request: https://projects.blender.org/blender/blender/pulls/130970
aa83738d44