Support movableContentOf across protocol

[JakeWharton]

Currently, due to #1899, we crash when the detached subtree is re-added because children IDs clash. Even once that's fixed, we need to detect the fact that it's a reused node (probably with something like a removed=true boolean on the node) and upon re-insertion, walk its children and add them all to the map.

Once guest-side is fixed, we basically have the same problem host-side. It has the map leak that will cause the crash (#1900), but once that's fixed we need the same tree walk on re-attach.

It is unlikely that we will retain the host-side nodes across recompositions. If you are just moving the content in a single composition, however, we can send a new boolean in the remove op to signal that this is a detach with a future op performing re-attach via insertion. This will keep all nodes in the map, and only perform a re-parenting from the old children to the new children.