By Kevin Yao and Langi Chiang BEIJING (Reuters) - China's economic recovery unexpectedly stumbled in the first three months of 2013 with slowing factory output and investment spending forcing analysts to start slashing full-year forecasts despite official insistence that the outlook was favorable. The world's second-biggest economy grew 7.7 percent in the first quarter from a year ago, slower than 7.9 percent hit in Q4 2012, below the Reuters consensus forecast of 8. ...