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
* Caches an element that derives from this element.
	 * <p>
	 * Part of: Inheritance system.
	 * 
	 * @param child
	 *            The new derived element.
	 */
	protected void addDerived( DesignElement child )
	{
		if ( derived == null )
			derived = new ArrayList( );
		assert child != null;
		assert child.getExtendsElement( ) == this;
		assert !derived.contains( child );
		assert child.getDefn( ) == getDefn( );
		derived.add( child );
	}
	/**
	 * Removes a cached element that derives from this element.
	 * <p>
	 * Part of: Inheritance system.
	 * 
	 * @param child
	 *            The old derived element.
	 */
	protected void dropDerived( DesignElement child )
	{
		assert derived != null;
		assert child != null;
		assert child.getExtendsElement( ) == this;