ST. PETERSBURG, Russia (Reuters) - Japanese Prime Minister Shinzo Abe and Chinese President Xi Jinping shook hands and exchanged words politely on Thursday on the sidelines of a G20 summit meeting, a Japanese official said, in an unexpected show of cordiality amid strained ties. Relations between the world's second- and third-largest economies have been troubled for months because of a row over tiny islands in the East China Sea known as the Senkaku in Japan and the Diaoyu in China. There are also disputes over the countries' wartime past. ...