Bug 146761 All structure properties change can't be restore
Steps to reproduce: 1.New a report, extends lib.table 2.Add a structure property(such as filter, column binding...) on table 3.Choose the table, click "Restore Property" Expected result: Local propery is removed Actual result: Local property can't be removed so that "Restore Property" is always enabled
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/api/GroupPropertyHandle.java
*/
	public IElementPropertyDefn getPropertyDefn( )
	{
		return this.propDefn;
	}
	/**
	 * Clears the value of the property on every element.
	 * 
	 * @throws SemanticException
	 *             If the value cannot be cleared.
	 */
	public void clearValue( ) throws SemanticException
	{
		setValue( null );
	}
	/**
	 * Returns the element reference value list if the property is element
	 * referenceable type. The list of available elements are sorted by their
	 * names lexicographically.
	 * 
	 * @return list of the reference element value.
	 */
	public List getReferenceableElementList( )
	{
		List elements = handle.getElements( );
		if ( elements == null || elements.isEmpty( ) )
			return Collections.EMPTY_LIST;
		DesignElementHandle element = (DesignElementHandle) elements.get( 0 );
		PropertyHandle propHandle = element.getPropertyHandle( propDefn
				.getName( ) );