Notify QuickDialogController when tableViewCell will be deleted

Right now you can swipe to delete if you configure a QSortingSection to allow it, but there's no way for the underlying QuickDialogController to be notified of this event (there's really no way, tried all manner of KVO!).

With this change QuickDialogController will now have a willDeleteElement: method, so data sources can be updated after table cells are deleted.