The name `id_root` was not descriptive, and was just a hold-over from the
equivalent (now deprecated) property on the Action itself. `target_id_type`
is more clear, reflecting that this is the type of ID the Slot is intended
to animate.
This PR also renames the corresponding `id_root_icon` to
`target_id_type_icon`.
Note that this PR updates the GLTF import/export core addon to adhere to
these name changes as well.
Pull Request: https://projects.blender.org/blender/blender/pulls/133164
9f2ab9cba0