Patch: force draggable's containment property to update during drag

The containment property for a draggable cannot be changed while the element is being dragged. This patch resets the containment during the drag.

Use case: a draggable element mimics a slider on a horizontal graph that can be stretched. When the draggable reaches one end of the graph, the graph widens. The slider is custom contained to the left half of the graph.

If the slider element widens the graph, its containment area will appear to shrink.

With this patch, client code can recalculate the containment area during the drag so that the containment area is maintained at half of the total width.

Patch for ui.draggable.js.

        _mouseDrag: function(event, noPropagation) {

            /// PATCH START ///
            var o = this.options;
            //Set a containment if given in the options
            if (o.containment)
                this._setContainment();
            //Recache the helper size
            this._cacheHelperProportions();
            /// PATCH END ///

            //Compute the helpers position
            this.position = this._generatePosition(event);
            this.positionAbs = this._convertPositionTo("absolute");