Must Articles 56 and 58 TEC, now Articles 63 and 65 TFEU, be interpreted as meaning that the cash-flow disadvantage resulting from the application of withholding tax to dividends paid to loss-making non-resident companies, while loss-making resident companies are not taxed on the amount of the dividends they receive until the year when, if at all, they return to a surplus, constitutes in itself a difference in treatment characterising a restriction on the free movement of capital?