Chicago’s music is timeless.Heavily jazz-influenced, with tinges of classical in the slower numbers, the songs, some now more than 50 years old, likely still will be popular and as evocative 50 years from now.That was the message from Chicago’s concert Friday headlining Bloomsburg Fair’s grandstand.In a far-too-short (more about that later) 88-minute, 18-song show, there were