EXHIBITION:Bono had discovered high heels, Edge didn't wear hats and there were photoshoot japes with some unforgettable fire extinguishers. Next week a selection of rare U2 photographs will go on show at the Little Museum of Dublin. These images, never been exhibited before, capture a teenage band on the verge of world domination. 'U2 1978-81' will be opened by Paul McGuinness on Thursday. Here, the photographer Patrick Brocklebank recalls those heady days in Dublin