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
/**
	 * Value will be returned as string only if all values of this property are
	 * equal within the collection of elements.
	 * 
	 * @return The value as string if all the element values for the property
	 *         are equal. Return null, if elements have different value for the
	 *         property.
	 * @see SimpleValueHandle#getStringValue()
	 */
	public String getStringValue( )
	{
		if ( !shareSameValue( ) )
			return null;
		
		
		List elements = handle.getElements( );
		return ( (DesignElementHandle) elements.get( 0 ) )
				.getStringProperty( propDefn.getName( ) );
	}
	/**
	 * Value will be returned as string only if all values of this property are
	 * equal within the collection of elements and one of them has a local
	 * value.
	 * 
	 * @return The value as string if all the element values for the property
	 *         are equal and one of them has a local value. Return null, if
	 *         elements have different value for the property or none of them
	 *         has a local value.
	 */
	public String getLocalStringValue( )