Link operation is currently unused (it was previously there for undo,
but we have removed the undo feature for now). The chldActor and chldCtr
columns were only used by link operations, so they can be removed too.
However, for cursors we also need an (actor, counter) pair to reference
the list element in question, so I'm renaming these columns to refActor
and refCtr and will use them for cursor storage.

For now it just stores the elemId referenced by the cursor in the
backend, updates the frontend-backend protocol to handle cursors, and
instantiates Cursor objects in the frontend.

Does not actually perform the conversion from elemId to index yet;
that's next.