(Adds analyst comment, details, paragraphs 5-6, 14-15, 17-19) * China's Q3 GDP grows 4.8% vs Q2's 5.2%, in line with forecast * Sept data show weaker retail sales, stronger factory output * Property downturn, trade tensions weigh on demand, confidence * Investors look to fresh stimulus as growth slows * Party meeting awaited for clues on efforts to rebalance economy By Ellen Zhang and Kevin Yao BEIJING, Oct 20 (Reuters) – China's economic growth slowed to the weakest pace in a year in the third quarter, as a property crisis and trade tensions hurt demand, leaving policymakers with the daunting challenge of rebalancing GDP drivers towards greater consumption over time. Authorities have unveiled modest stimulus measures this year, supported by resilient exports and strong stock markets, but

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