Drag-and-drop is not needed for many widgets that currently use this widget, so maybe it would be better to create a subclass of Uize.Widget.Drag called Uize.Widget.DragAndDrop or Uize.Widget.Drag.Droppable or something, to have this extra functionality? Right now, sliders and resizable dialogs and marquees and such use this class. Do they need the drop functionality?