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/api/DesignElementHandle.java
* 
	 * @param propName
	 *            the name of the property to get. Can be a system-defined or
	 *            user-defined property name.
	 * 
	 * @return the property binding, or null if the overridden value is not set
	 */
	public String getPropertyBinding( String propName )
	{
		PropertyBinding propBinding = module.findPropertyBinding(
				getElement( ), propName );
		if ( propBinding == null )
			return null;
		return propBinding.getValue( );
	}
	/**
	 * Sets the mask of the specified property.
	 * 
	 * @param propName
	 *            the property name to get. Can be a system-defined or
	 *            user-defined property name.
	 * 
	 * @param value
	 *            the overridden value
	 * 
	 * @throws SemanticException
	 *             if the maskValue is not one of the above.
	 */
	public void setPropertyBinding( String propName, String value )
			throws SemanticException