FRANKFURT (Reuters) - The European Central Bank promised an indefinite supply of fresh asset purchases and cut interest rates deeper into negative territory on Thursday in an effort to prop up the ailing euro zone economy that was cheered by financial markets.Coming in the final weeks of ECB President Mario Draghi's mandate, the moves set the stage for likely easing next week by the U.S. Federal Reserve and Bank of Japan aimed at supporting a world economy increasingly characterised by low..