Bug 181121 Model should provide utility method for cube to get correspond dimension key.
In model cube handle, the relation of cube data set and dimension data sets are managed by a series of dimension condition handles.We need model to add utility method so that we can easily get corresponding dimension data set column names according to given cube dimension keys.
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/core/Structure.java
* 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
	 */
	private void doUpdateReference( ElementRefValue oldRef,
			ElementRefValue newRef, PropertyDefn prop )
	{
		ReferenceableElement target;
		DesignElement me = getContextElement( );;
		String propName = getContextPropertyName( );
		
		
		if ( oldRef != null )
		{
			target = oldRef.getTargetElement( );
			if ( target != null && me != null && propName != null )
				target.dropClient( me, propName );
		}
		
		
		
		if ( newRef != null )
		{