By Tobias Carroll For decades, the island of Mustique — located within Saint Vincent and the Grenadines — was a deeply desirable spot for a host of world-famous personalities. Princess Margaret was one of the island’s most high-profile admirers, as was Mick Jagger, and it’s still a choice destination for some — including Boris Johnson, whose travel plans there last year sparked a minor scandal. In recent years, however, Mustique’s star has dwindled somewhat compared to that of nearby Bequia, which has its own celebrity admirers — including Bob Dylan. What’s behind this relative shift in popula…