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
List elements = handle.getElements( );
		return ( (DesignElementHandle) elements.get( 0 ) )
				.getDisplayProperty( propDefn.getName( ) );
	}
	/**
	 * Set the object value on a group of elements. This operation will be
	 * executed within a transaction, it will be rollbacked if any set operation
	 * failed.
	 * 
	 * @param value
	 *            the object value to set
	 * @throws SemanticException
	 *             if the property is undefined on an element or the value is
	 *             invalid.
	 * @see PropertyHandle#setValue(Object)
	 */
	public void setValue( Object value ) throws SemanticException
	{
		assert handle.getModule( ) != null;
		ActivityStack actStack = handle.getModule( ).getActivityStack( );
		actStack.startTrans( );
		try
		{
			for ( Iterator iter = handle.getElements( ).iterator( ); iter
					.hasNext( ); )
			{
				DesignElementHandle elemHandle = (DesignElementHandle) iter
						.next( );