In the late 20th century, a handful of people initiated an effort to revitalize the communities on the islands of Japan’s Seto Inland Sea, whose populations were shrinking as young people left for jobs and cities. The project — to make the islands centers of modern art — took off and paid off. Today, several of the islands have innovative art museums and year-round art installations that annually draw a million tourists. Every three years, a dozen of them host the Setouchi Triennale, a festival exhibiting the work of artists from all over the world. It’s happening now and continues for most of 2019….