Removed some code duplication

For backward-compatibility, add a new 'fragments' property to the
fragmentshown and fragmenthidden events. The 'fragment' property still
returns the first fragment.

Previously navigatePrev() was changing indexh and indexv; instead should
call slide(h, v) and let it change indexh and indexv.

When multiple elements are specified with the same data-fragment-index,
they are all shown or hidden at once.