Revised method impl for EnergyOfferGrid that composes a number of AbstractValidatableColumns into a CompositeValidatableColumn

EnergyOffer.addPriceMwColumn

protected void addPriceMwColumn(final int colIndex, DisplayMode currentDisplayMode) {

        // Construct a composite cell for energy offers that includes a pair of text inputs
        final List<HasCell<EnergyOfferDTO, String>> columns = new ArrayList<
                HasCell<EnergyOfferDTO, String>>();

        // this DTO is passed along so that price and mw values for new entries are kept together
        final OfferPriceMwPair newOfferPriceMwPair = new OfferPriceMwPair();

        // Price
        final Column<EnergyOfferDTO, String> priceColumn = generatePriceColumn(colIndex, newOfferPriceMwPair, currentDisplayMode);
        columns.add(priceColumn);

        // MW
        final Column<EnergyOfferDTO, String> mwColumn = generateMwColumn(colIndex, newOfferPriceMwPair, currentDisplayMode);
        columns.add(mwColumn);

        // Composite
        final WrapperCell<EnergyOfferDTO> priceMwColumnInnards = generateWrapperCell(columns);

        final CompositeValidatableColumn<EnergyOfferDTO> priceMwColumn = new CompositeValidatableColumn<EnergyOfferDTO>(priceMwColumnInnards);

        final StringBuilder colHeader = new StringBuilder();
        colHeader.append(UiMessages.INSTANCE.price_mw_header()).append(" ").append(String.valueOf(colIndex + 1));

        addColumn(priceMwColumn, colHeader.toString());
        setColumnWidth(priceMwColumn, 7, Unit.EM);
        setColumnHorizontalAlignment(priceMwColumn, HasHorizontalAlignment.ALIGN_RIGHT);
    }