LONDON (Reuters) - European stocks stumbled and the dollar dropped to three-month lows on Monday as hopes waned for progress in Sino-U.S. trade talks at this week's G20 meeting and fears grew of a confrontation with Iran.Investors are waiting to see if Presidents Donald Trump and Xi Jinping can de-escalate a trade war that is damaging the global economy and souring business confidence. The leaders will meet during a G20 summit in Japan.Pan-European STOXX 600 fell 0.2% , reflecting losses in..