Support recursive collections based on ImmutableList

[AArnott]

This work in progress under the ProjectTreeWork branch.

CC: @jviau

[AArnott]

Currently the reordering detection that would report to ChangesSince doesn't work. When sorted, an item changes position when that item changes its properties in some way. But in an ordered collection, no change to the item itself is required to reorder. So we need to figure out how to activate that code path without relying on the reordered items themselves changing.