Bug 267117 [JFace] Provide possibility for subclasses to check if a column is visible in ViewerRow
Currently this is decided by checking with width but this makes Nebula-Grid fail.
bundles/org.eclipse.jface/src/org/eclipse/jface/viewers/ViewerRow.java
* 
	 * @param column
	 * @return {@link ViewerCell} or <code>null</code> if the index is negative.
	 */
	public ViewerCell getCell(int column) {
		if (column >= 0)
			return new ViewerCell((ViewerRow) clone(), column, getElement());
		return null;
	}
	/**
	 * Get the Control for the receiver.
	 * 
	 * @return {@link Control}
	 */
	public abstract Control getControl();
	/**
	 * Returns a neighboring row, or <code>null</code> if no neighbor exists in
	 * the given direction. If <code>sameLevel</code> is <code>true</code>, only
	 * sibling rows (under the same parent) will be considered.
	 * 
	 * @param direction
	 *            the direction {@link #BELOW} or {@link #ABOVE}
	 * 
	 * @param sameLevel
	 *            if <code>true</code>, search only within sibling rows
	 * @return the row above/below, or <code>null</code> if not found
	 */
	public abstract ViewerRow getNeighbor(int direction, boolean sameLevel);
	/**
	 * The tree path used to identify an element by the unique path