MILAN (Reuters) - Eyewear billionaire Leonardo Del Vecchio has requested clearance from the European Central Bank to double his stake in Mediobanca , raising the prospect of far-reaching changes in Italy's financial world.Mediobanca has not had a single non-banking shareholder owning more than 10% since it was founded in 1946. Del Vecchio's move has fuelled speculation his ultimate goal could be a shake-up involving Generali , Italy's biggest insurer in which Mediobanca is the single largest..