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
{
		if ( !shareSameValue( ) )
			return null;
		List elements = handle.getElements( );
		for ( int i = 0; i < elements.size( ); i++ )
		{
			DesignElementHandle element = (DesignElementHandle) elements
					.get( i );
			Object value = element.getElement( ).getLocalProperty(
					element.getModule( ), propDefn );
			String localValue = propDefn.getStringValue( element
					.getEffectiveModule( ), value );
			if ( localValue != null )
				return localValue;
		}
		return null;
	}
	/**
	 * Value will be returned as string only if all values of this property are
	 * equal within the collection of elements. The value return are localized.
	 * 
	 * @return The localized 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#getDisplayValue()
	 */
	public String getDisplayValue( )
	{
		if ( !shareSameValue( ) )
			return null;