Previously it was possible to make the type prefix of a Slot's identifier get
out-of-sync with its actual target ID type, by setting the identifier via
Python.
This PR changes `Slot.identifier` assignment to ensure that the type prefix is
set to match `target_id_type`. This now makes it impossible for the identifier
prefix and `target_id_type` to get out of sync, since this API previously was
the only way to accomplish that.
When the prefix that the user attempts to set doesn't match the `target_id_type`
of the Slot, a warning is issued telling the user about the mismatch and that
the identifier has been set with the correct prefix instead.
Pull Request: https://projects.blender.org/blender/blender/pulls/133983
Nathan Vegdahl
f84197f0b9