Dolce & Gabbana said Monday that it has appointed the former Gucci boss Stefano Cantino as its co-chief executive to flank CEO Alfonso Dolce.     The move comes after it emerged last week that co-founder Stefano Gabbana stepped down as the president of the Italian fashion house in January amid a shakeup as the company negotiates the refinancing of around 450 million euros of debt with banks.     Alfonso Dolce, the brother of the group's other co-founder Domenico Dolce, replaced Gabbana as