Check delegate for swipe after checking screen edges

This new delegate method was exactly what I was looking for :)

However it now pre-empts the `paneDragRequiresScreenEdgePan` preference, which I was previously relying on. I've tweaked it to call the delegate after establishing it is a valid screen edge pan. If `paneDragRequiresScreenEdgePan` is `NO`, it will still fall back on the delegate first.