1. DragDestination
dragDestinationActionMaskForDraggingInfo : WebKit asks for the dragging types which are allowed by client
willPerformDragDestinationAction : WebKit allows clients to handle this action first. However in my mind, this interface should return a boolean value to indicate whether WebKit should handle this action after that
2. DragSource
dragSourceActionMaskForPoint : WebKit asks for the dragging types which are allowed by client
willPerformDragSourceAction : WebKit inform client that a drag will be handled
DoDragDrop : Clients can handle this function to handle drag and drop, this function is in WebUIDelegatePrivate. Or clients can hook ::doDragDrop to handle this operation.
