Bug 167323 [Viewers]Open AbstractTreeViewer for external widget implementors
As reported by Ross Camara AbstractTreeViewer would work for nebula if we would open the API a bit. Boris and I decided that we will do this in 2 steps: 1. open the API 2. moving the Virtual-Support bits from TreeViewer to AbstractTreeViewer when the nebula grid provides virtual support.
bundles/org.eclipse.jface/src/org/eclipse/jface/viewers/AbstractTreeViewer.java
*            that labels for existing elements are unchanged.
	 * @since 2.1
	 */
	private void updateChildren(Widget widget, Object parent,
			Object[] elementChildren, boolean updateLabels) {
		
		if (widget instanceof Item) {
			Item ti = (Item) widget;
			if (!getExpanded(ti)) {
				
				
				boolean needDummy = isExpandable(ti, null, parent);
				boolean haveDummy = false;
				
				Item[] items = getItems(ti);
				for (int i = 0; i < items.length; i++) {
					if (items[i].getData() != null) {
						disassociate(items[i]);
						items[i].dispose();
					} else {
						if (needDummy && !haveDummy) {
							haveDummy = true;
						} else {
							items[i].dispose();
						}
					}
				}
				if (needDummy && !haveDummy) {
					newItem(ti, SWT.NULL, -1);
				}