Bug 124620 Referenced Parameter icon should be invalid if the parameter in library is deleted
Description: Referenced parameter icon should be invalid if the source parameter in library is deleted. Steps to reproduce: 1.Create a library. Insert a report scalar parameter. 2.Create a report, use the above library. In the library explorer, pull the parameter to the data explorer view of report. 3.Open the library file, delete the parameter. Then reopen the report design file, the icon of parameter seems nothing happened. But if those operations happened in dataset, the icon of dataset in the report will be invalid. It is acceptable. Expected Result: The icon of referenced parameter should be invalid.
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/core/DesignElement.java
* include elements that directly extend from this one, along with elements
	 * that extend from those, and so on recursively.
	 * <p>
	 * Part of: Inheritance system.
	 * 
	 * @return The list of descendents.
	 */
	public List getDescendents( )
	{
		ArrayList list = new ArrayList( );
		gatherDescendents( list );
		return list;
	}
	/**
	 * Builds up a list of the elements that extend from this one, directly or
	 * indirectly.
	 * <p>
	 * Part of: Inheritance system.
	 * 
	 * @param list
	 *            The list of descendents.
	 */
	public void gatherDescendents( ArrayList list )
	{
		if ( derived == null )
			return;
		for ( int i = 0; i < derived.size( ); i++ )
		{
			DesignElement child = (DesignElement) derived.get( i );
			list.add( child );