struct AddPartCmd

Add a node as a part of another node. The implications of being a part

rather than a child differ based on the type of the part. However, one

implication is constant: removing all of a node's children (e.g. via

DetachChildrenCmd) does not affect its parts. This is similar to the

"shadow DOM" in a web browser: the controls of a

<video

> element are

implemented as using the shadow DOM, and do no show up amongst the children

of that element.

Constraints:

- `id` refs a Node with the has_parts characteristic.

- `part_id` refs any Node.

Discussion:

The part node is first removed from its existing parent, as if DetachCmd

was applied first.