HONG KONG/NEW YORK, July 31 (Reuters) - For all the excitement whipped up in China's markets by the Politburo last week, foreign investors say policymakers' words will have to be matched by substantive action to clean up an ailing property sector before confidence recovers. The sector accounts for a quarter of China's economy, yet developers' debts are sliding deeper into distressed territory, with repayment problems mounting while sales crumble. When the Politburo signalled that there would be changes to real estate policy, along with other measures to boost an economy, it ignited the biggest one-day buying spree in China's stock...