Presenter mode should show the next animation rather than the next slide

[vlsi]

Description

Case: a slide with 3 appear items.
The current slide in the presenter view shows only the currently shown items (which is right).
However, the next slide preview shows the next slide which is completely irrelevant. The next slide preview should show what will the screen look like after advancing one item.

Sample: https://raw.githack.com/FormidableLabs/spectacle/master/examples/one-page.html?slide=4&slideElement=0

The slide is

Animated Elements
1. Elements can animate in!
2. Just identify the order with the prop elementNum!
3. Out of order

The problem is "the next slide preview" does not tell the presenter that the items will appear out of order. It does not tell which items will appear. That limits the usability from the presenter view a lot.

Proposal

The next slide should render the slide with the next slide element.

Links / References

The suggestion is in line with PowerPoint and Keynote which shows "next animation" rather than "next slide".
On top of that, Keynote can optionally how the number of animations left.

This issue might be relevant to #644, however, I'm sure the next preview should show the next animation rather than the next slide ignoring all the current animations.