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/api/DesignElementHandle.java
{
		getElement( ).removeListener( obj );
	}
	/**
	 * Returns an iterator over the elements that derive from this one.
	 * 
	 * @return an iterator over the elements that derive from this one. Each
	 *         item returned by the iterator's <code>getNext( )</code> method
	 *         is of type {@link DesignElementHandle}.
	 */
	public Iterator derivedIterator( )
	{
		return new DerivedElementIterator( module, this );
	}
	/**
	 * Returns an iterator over the clients of this element. Useful only for
	 * styles. Returns a list of all the elements that use this style.
	 * 
	 * @return an iterator over the clients of this element. Each item returned
	 *         by the iterator's <code>getNext( )</code> method is of type
	 *         {@link DesignElementHandle}. Nothing will be iterated over an
	 *         element that is not <code>ReferenceableElement</code>.
	 */
	public Iterator clientsIterator( )
	{
		return new ClientIterator( this );
	}
	/**
	 * Returns a array of valid choices for a property.
	 *