TAIPEI, Taiwan Taiwan's former president, Ma Ying-jeou, will be setting off on Monday for the first visit to China by any sitting or former Taiwanese leader since China's civil war ended with the Nationalist government retreating to the island from the mainland in 1949.Though the 12-day visit by Mr. Ma, who was president from 2008 to 2016, is unofficial, it is likely to be watched closely at home and abroad for clues on how Beijing might seek to influence Taiwan, its democratic neighbor, ahead of a presidential election in January.