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
* (non-Javadoc)
	 * 
	 * @see org.eclipse.jface.viewers.ColumnViewer#getColumnViewerOwner(int)
	 */
	protected Widget getColumnViewerOwner(int columnIndex) {
		
		return null;
	}
		
	/**
	 * This implementation of {@link #getItemAt(Point)} returns null to ensure
	 * API backwards compatibility. Subclasses should override.
	 * 
	 * @since 3.3
	 */
	protected Item getItemAt(Point point) {
		return null;
	}
	
	/**
	 * This implementation of {@link #createViewerEditor()} returns null to ensure
	 * API backwards compatibility. Subclasses should override.
	 *
	 * @since 3.3
	 */
	protected AbstractViewerEditor createViewerEditor() {
		return null;
	}
}