minor changes and new approach to incremental slides

Here's the description of the commits:

1. "also show overview on Escape" — many javascript-based presentation frameworks show the overview on Esc key (e.g. flowtime.js, reveal.js, shower). dzslides doesn't and I originally thought that overview is not supported. Only later looking at the code I saw that it actually does work. I've made dzslides to show overview on Esc to avoid similar confusions in the future.

2. redo incremental lists: three commits. This changes the approach to incremental lists. Instead of putting "incremenal" in the UL element, one now puts "next" on every LI element. Benefits:
 * more flexible. "next" can be on any element, not only on LI. So any part of the slide can be incremental (there's an example with "next" on P)
 * simpler styles: only one rule to specify the behavior, not four (for UL, for UL:active, for LI, and for LI:aria-selected)
 * because any element can be incremental, the behavior can also be arbitrary styled. There's an example where text highlighting appears incrementally (color:yellow), while the text itself doesn't disappear or appear.
 * another feature: incremental elements can optionally have sequence numbers (using next-order attribute). They will appear in that order. There's an example too.
 * incremental elements behave as in shower — their incremental behavior happens only once, further back()/forward() don't make elements to disappear again. One needs to reload the page to reset incremental elements. This is very important — after a slide is completely shown, you can quickly go back and forth to any slide, answering questions of attendees, and and incremental behavior would only be annoying. I hated that in every my talk (libreoffice, Slidy, etc) — until I saw how shower does it.

3. other commits are pretty much self-explanatory