Notes view glitches when using multiple elements with same data-fragment-index

PR #435 added the very useful possibility to assign the same data-fragment-index to multiple fragments and show/hide them at once.

This breaks the notes view, however, since each fragment triggers a fragmenshown / fragmenthidden event. The notes plugin calls Reveal.nextFragment() / Reveal.prevFragment() on each received event, so the notes view progresses too fast. I assume this also affects the notes-server plugin, but I didn't check.

The notes plugin could be changed to filter out additional events. Another is to create a single event for all displayed fragments with the same index. This pull request does just that.

For backwards-compatibility, the fragment property of the event still returns a single fragment. A new property (fragments) contains all fragments that were displayed.

What do you think?