After a successful career in finance, Darla Moore returned to her hometown, Lake City, S.C., and investigated ways she could help improve its fortunes. “I didn’t walk in and say, ‘I’m back, and I’ve brought a bunch of money, so let’s fix ourselves up,’” she said.Many once-great towns in America have lost their luster. If a philanthropist wants to help turn one around, the complex challenges call for a community-based focus.