Add Property "SourceIndex" also to IDragInfo (same as in IDropInfo)

[knoxi]

Changes in IDragInfo:

        /// <summary>
        /// Gets the position from where the item was dragged.
        /// </summary>
        int SourceIndex { get; }

Changes in DragInfo:

        /// <summary>
        /// Gets the position from where the item was dragged.
        /// </summary>
        /// <value>The index of the source.</value>
        public int SourceIndex { get; private set; }

add following code in the statement "if (itemParent != null)":

this.SourceIndex = itemParent.ItemContainerGenerator.IndexFromContainer(item);