Most of the old Animato properties on an Action (e.g. FCurve list, Channel
Groups) already act as proxies for the data for the first slot in the first
strip of the first layer. (Say that three times fast!) However, this was not yet
the case for `Action.id_root`.
This PR changes `Action.id_root` to act as a proxy for the first Slot's
`target_id_type` property, both for reading and writing.
If the Action has no Slots, then reading always returns 'UNSPECIFIED', and
writing will create a Slot and set its `target_id_type`.
Note that the ability to write to the first Slot's `target_id_type` via
`Action.id_root` conflicts with `target_id_type` supposedly only being writable
when it's still 'UNSPECIFIED' (#133883). Although that's certainly a little
weird, practically speaking this doesn't break anything for now, and is a
temporary kludge to keep `id_root` working until we can remove it in Blender
5.0. `id_root` will be removed entirely in 5.0, resolving this inconsistency.
Pull Request: https://projects.blender.org/blender/blender/pulls/133823
eda2f11f7a