Bug 177504 Engine doesn't provide SubQuery for extended item
Engine doesn't provide SubQuery for extended item, so chart can't use container's binding to execute the query. If possible, engine could provide an option for extended item to choose whether engine could create the query, so extended item won't create SubQuery by itself.
engine/org.eclipse.birt.report.engine/src/org/eclipse/birt/report/engine/presentation/LocalizedContentVisitor.java
case IReportItemPresentation.OUTPUT_AS_CUSTOM :
				IDataContent dataObj = reportContent.createDataContent( content );
				dataObj.setValue( output );
				processData( dataObj );
				return dataObj;
			case IReportItemPresentation.OUTPUT_AS_HTML_TEXT :
				content.setRawType( IForeignContent.HTML_TYPE );
				content.setRawValue( output.toString( ) );
				return content;
			case IReportItemPresentation.OUTPUT_AS_TEXT :
				ITextContent textObj = reportContent.createTextContent( );
				textObj.setText( output.toString( ) );
				return textObj;
				
			default :
				assert false;
				logger.log( Level.WARNING, "unsupported output format:{0}", 
						new Integer( type ) );
		}
		return content;
	}
	/**
	 * read the content of input stream.
	 * 
	 * @param in
	 *            input content
	 * @return content in the stream.
	 */
	static protected byte[] readContent( InputStream in )
	{