For 30 years, Mark Dew has played the piano and sang classic tunes at the Hideaway (5900 Arsenal Street, 314-645-8822). In many ways, performer and place have become synonymous — so much so that when news broke that the longtime owner of the beloved dive had died, the first question for many regulars wasn't "what about the Hideaway?" but rather "what about Mark Dew?"Patrons need not have worried.…