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
*            the number of items
	 * 
	 * @since 3.3
	 */
	protected abstract void doSetItemCount(int count);
	/**
	 * Returns a (possibly empty) array of TableItems which are the items in the
	 * receiver.
	 * 
	 * @return the items in the receiver
	 * 
	 * @since 3.3
	 */
	protected abstract Item[] doGetItems();
	/**
	 * Returns the number of columns contained in the receiver. If no columns
	 * were created by the programmer, this value is zero, despite the fact that
	 * visually, one column of items may be visible. This occurs when the
	 * programmer uses the table like a list, adding items but never creating a
	 * column.
	 * 
	 * @return the number of columns
	 * 
	 * @since 3.3
	 */
	protected abstract int doGetColumnCount();
	/**
	 * Returns the column at the given, zero-relative index in the receiver.
	 * Throws an exception if the index is out of range. Columns are returned in
	 * the order that they were created. If no TableColumns were created by the