searchDisplay: Cache result display actors

searchDisplay: Cache result display actors

When we create a result actor, cache it, so it can be used for
subsearches of the same initial. For now, to keep memory usage
and the stage graph relatively clean, don't persist the actors
across searches, but maybe we should do this in the future.

This also means that we don't query getResultMetas for items
that we've seen in the same initial search.

https://bugzilla.gnome.org/show_bug.cgi?id=704912