also show overview on Escape
redo incremental lists

use 'next' class on every node, not 'incremental' on the list as a whole
* more flexible
  'next' can be used on anything (not only lists),
  and can behave any way I want
* style needs to be specified only once, not four times
  (for [active], not [active], on the list and
  [aria-selected] and not [aria-selected] on the list element)
* make next elements to appear only once (as in shower).
  when answering questions after the talk and quickly paging through
  slides, this interaction only annoys, not helps.
try incremental text hightlighting
incremental elements can appear in any arbitrary order
in overview mode: don't play video and don't do incremental stuff
optional header/footer on every page
todo