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/metadata/ExtendsPropertyType.java
PropertyValueException.DESIGN_EXCEPTION_INVALID_VALUE,
				PropertyType.ELEMENT_REF_TYPE );
	}
	/**
	 * Returns the referenced element name if the input value is an
	 * <code>ElementRefValue</code>, return <code>null</code> if the value
	 * is null.
	 */
	public String toString( Module module, PropertyDefn defn, Object value )
	{
		ElementRefValue refValue = (ElementRefValue) value;
		if ( refValue == null )
			return null;
		return refValue.getQualifiedReference( );
	}
	/**
	 * Validates the element value.
	 * 
	 * @param module
	 *            report design
	 * @param targetDefn
	 *            definition of target element
	 * @param target
	 *            target element
	 * @return the resolved element reference value
	 * @throws PropertyValueException
	 *             if the type of target element is not that target definition.
	 */
	private ElementRefValue validateElementValue( Module module,
			PropertyDefn targetDefn, DesignElement target )