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
} while ( e != null );
		return null;
	}
	/**
	 * Implements to cache a back-pointer from a referenced element. This
	 * element has an element reference property that can point to another
	 * "referencable" element. To maintain semantic consistency, the referenced
	 * element maintains a list of "clients" that identifies the elements that
	 * refer to it. The client list is used when the target element changes
	 * names or is deleted. In these cases, the change automatically updates the
	 * clients as well.
	 * <p>
	 * References can be in two states: resovled and unresolved. An unresolved
	 * reference is just a name, but the system has not yet identified the
	 * target element, or if a target even exists. A resolved reference caches a
	 * pointer to the target element itself.
	 * 
	 * @param oldRef
	 *            the old reference, if any
	 * @param newRef
	 *            the new reference, if any
	 * @param prop
	 *            definition of the property
	 */
	protected void updateReference( ElementRefValue oldRef,
			ElementRefValue newRef, ElementPropertyDefn prop )
	{
		ReferenceableElement target;