More sane 'default' for classPrefix & optional event.preventDefault()

Change 1: since we wanted a blank class prefix I changed the defaults so that the default classPrefix ('m-') is set only when the option is *undefined*, not a blank string.

Change 2: We had a carousel item with controls (embedded Vimeo video); so we needed to allow event defaults.   To maintain correct carousel behaviour we caught the events further down the DOM tree.