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
if (parentElementOrTreePath != null
				&& comparator instanceof TreePathViewerSorter) {
			TreePathViewerSorter tpvs = (TreePathViewerSorter) comparator;
			
			result = (Object[]) result.clone();
			TreePath path = null;
			if (parentElementOrTreePath instanceof TreePath) {
				path = (TreePath) parentElementOrTreePath;
			} else {
				Object parent = parentElementOrTreePath;
				Widget w = internalGetWidgetToSelect(parent);
				if (w != null) {
					path = internalGetSorterParentPath(w, comparator);
				}
			}
			tpvs.sort(this, path, result);
		} else if (comparator != null) {
			
			result = (Object[]) result.clone();
			comparator.sort(this, result);
		}
		return result;
	}
	/*
	 * (non-Javadoc)
	 * 
	 * @see org.eclipse.jface.viewers.StructuredViewer#getFilteredChildren(java.lang.Object)
	 */
	protected Object[] getFilteredChildren(Object parentElementOrTreePath) {
		Object[] result = getRawChildren(parentElementOrTreePath);