Bug 135284 [Viewers] API: need to add methods to StructuredViewer to support ViewerComparator
bundles/org.eclipse.jface/src/org/eclipse/jface/viewers/StructuredViewer.java
*            the transfer types that are supported by the drop operation
	 * @param listener
	 *            the callback that will be invoked after the drag and drop
	 *            operation finishes
	 * @see org.eclipse.swt.dnd.DND
	 */
	public void addDropSupport(int operations, Transfer[] transferTypes,
			final DropTargetListener listener) {
		Control control = getControl();
		DropTarget dropTarget = new DropTarget(control, operations);
		dropTarget.setTransfer(transferTypes);
		dropTarget.addDropListener(listener);
	}
	/**
	 * Adds the given filter to this viewer, and triggers refiltering and
	 * resorting of the elements.
	 * 
	 * @param filter
	 *            a viewer filter
	 */
	public void addFilter(ViewerFilter filter) {
		if (filters == null) {
			filters = new ArrayList();
		}
		filters.add(filter);
		refresh();
	}
	/**
	 * Asserts that the given array of elements is itself non- <code>null</code>
	 * and contains no <code>null</code> elements.