Bug 151959 inappropriate screen operation
steps to reproduce: - create a report - add a grid item (3x3) - switch to Outline view - expand the grid item to see its rows and cells - select the grid item, and drag it into any of its own cell expected behavior: - is it an invalid screen operation? if it is, it should be banned actual behavior: - the grid item was disappeared surprisingly
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/core/DesignElement.java
*            the listener to register
	 */
	public void addListener( Listener obj )
	{
		if ( listeners == null )
			listeners = new ArrayList( );
		if ( obj != null && !listeners.contains( obj ) )
			listeners.add( obj );
	}
	/**
	 * Removes a listener. The listener is removed from the list of listeners.
	 * If the item is not in the list, then the request is silently ignored.
	 * <p>
	 * Part of: Notification system.
	 * 
	 * @param obj
	 *            the listener to remove
	 */
	public void removeListener( Listener obj )
	{
		if ( listeners == null )
			return;
		int posn = listeners.indexOf( obj );
		if ( posn != -1 )
			listeners.remove( posn );
	}
	/**
	 * Removes all listeners on this element.
	 * <p>
	 * Part of: Notification system.