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
if ( oldRef != null )
		{
			target = oldRef.getTargetElement( );
			if ( target != null )
				target.dropClient( this );
		}
		
		
		
		if ( newRef != null )
		{
			target = newRef.getTargetElement( );
			if ( target != null )
				target.addClient( this, prop.getName( ) );
		}
	}
	/**
	 * Implements to cache a back-pointer from a referenced structure. This
	 * element has a structure reference property that can point to another
	 * "referencable" structure. To maintain semantic consistency, the
	 * referenced structure maintains a list of "clients" that identifies the
	 * elements that refer to it. The client list is used when the target
	 * structure 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 structure, or if a target even exists. A resolved reference caches
	 * a pointer to the target structure itself.
	 *