Preserve PCD/subscription order in merge

[artwyman]

This is a follow-on to #1342.

The simple merge algorithm in #1342 merges both PCDs and subscriptions by starting with the remote copy (downloaded from server) and adding any missing objects (with unique IDs) from the local copy. The objects "added" are thus moved to the end of the default display order. This is user-visible, and may not always match expectations, though admittedly it's a pretty small flaw.

We could be more careful about preserving order, e.g. trying to insert a PCD in the "remote" collection at the same point in folder order where it appears in the "local" collection. A more robust form of merging such as CRDTs would also handle this case implicitly.