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
*            the tree expansion event
	 * @see ITreeViewerListener#treeCollapsed
	 */
	protected void fireTreeCollapsed(final TreeExpansionEvent event) {
		Object[] listeners = treeListeners.getListeners();
		for (int i = 0; i < listeners.length; ++i) {
			final ITreeViewerListener l = (ITreeViewerListener) listeners[i];
			SafeRunnable.run(new SafeRunnable() {
				public void run() {
					l.treeCollapsed(event);
				}
			});
		}
	}
	/**
	 * Fires a tree expanded event. Only listeners registered at the time this
	 * method is called are notified.
	 * 
	 * @param event
	 *            the tree expansion event
	 * @see ITreeViewerListener#treeExpanded
	 */
	protected void fireTreeExpanded(final TreeExpansionEvent event) {
		Object[] listeners = treeListeners.getListeners();
		for (int i = 0; i < listeners.length; ++i) {
			final ITreeViewerListener l = (ITreeViewerListener) listeners[i];
			SafeRunnable.run(new SafeRunnable() {
				public void run() {
					l.treeExpanded(event);
				}