DragAndDropHelper::updateDropAction: use StatJob for remote URLs

When dragging onto tabs/Places from a remote URL, we don't process the QDropEvent immediately, but start a StatJob and process the event when it finishes.

Also, the result of the StatJob is cached for 30 seconds, to avoid starting duplicate jobs.

Since kwin in master doesn't show custom DnD cursors, you need to revert plasma/kwin@53863609 to test this.