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/AbstractTableViewer.java
if (virtualManager == null) {
			updateItem(internalCreateNewRowPart(SWT.NONE, index).getItem(),
					element);
		} else {
			virtualManager.notVisibleAdded(element, index);
		}
	}
	/**
	 * Create a new row
	 * 
	 * @param style
	 *            the style for the new row
	 * @param rowIndex
	 *            the index of the row or -1 if the row is appended at the end
	 * @return the newly created row
	 */
	protected abstract ViewerRow internalCreateNewRowPart(int style,
			int rowIndex);
	/**
	 * Adds the given element to this table viewer. If this viewer does not have
	 * a sorter, the element is added at the end; otherwise the element is
	 * inserted at the appropriate position.
	 * <p>
	 * This method should be called (by the content provider) when a single
	 * element has been added to the model, in order to cause the viewer to
	 * accurately reflect the model. This method only affects the viewer, not
	 * the model. Note that there is another method for efficiently processing
	 * the simultaneous addition of multiple elements.
	 * </p>
	 *