search-display: Try harder to use a correct drag actor source

Commit 429f809b fixed an exception in getDragActorSource(), but
the returned actor is only an approximation (e.g. in contrast to
the actual drag actor, it includes the label).
Try a bit harder to return the correct actor and only fall back to
the approximation when necessary.

https://bugzilla.gnome.org/show_bug.cgi?id=645990