Improve performance in big notebooks

[Masterxilo]

Cells that are not visible should not be rendered at all in the browser.

This way, we should be able to have a notebook with a billion cells (ignoring the issue of scrolling to a specific one...), because all that are not visible would be paged out to disk and represented by a single number describing their height (if that).

[Masterxilo]

It should not be that it matters for performance whether I have 1000 notebooks I open one after the other or one where I scroll to 1000 sections.

This is very not true in all implementations of notebooks I have seen so far (they're all in-memory), but good word processors, all movie players and good text editors (vim, more) & most games stream in just the stuff that is needed right now.

This is a basic technique that should be used in all places where items can grow "infinitely".