Creates a draggable system of docking elements, grouped into columns.
Creates a dock with numColumn columns.
Adds a new object to dock, resp. to its column with index colIndex. This object is represented by node mover and is to be dragged by node grabber. Grabber should be a child of mover.
var mover = Dom.create("div");
var grabber = Dom.create("img");
mover.appendChild(grabber);
var d = new Dock(3);
d.addObject(0, grabber, mover);