This week, the Boston Symphony will perform John Harbison’s overture to his opera based on Vladimir Nabokov’s groundbreaking and controversial novel, Lolita. What’s that you say? You didn’t know that Harbison had written an operatic version of the disturbing tale of teenage sexuality and seduction? Well, actually, he hasn’t. He wanted to, you understand, and actually got as far as sketching out the form and writing the overture, but then, the Catholic church in Boston, where Mr. Harbison lives, exploded into chaos with the sexual abuse scandal involving priests, and the composer couldn’t bring himself to finish the project as the dark comedy it is intended to be. Moreover, he concluded that Lolita simply cannot work in operatic form at all.