Math editing performance

[cben]

This is about editing math-heavy text in fully-initialized document; see #14 for init performance (though many improvements there would also help here).

• cache typeset DOM, reuse if text & token class didn't change.
• if changed, use jax methods like .Text(newmath)
  http://docs.mathjax.org/en/latest/typeset.html#manipulating-individual-math-elements
  • use .Rerender() if only token class changed.
• be more precise in area to be updated.

Could Hub.Update() do the job instead a custom caching scheme?!
http://docs.mathjax.org/en/latest/api/hub.html#Update

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Easy ways to test:

• select a big block and press Tab/Shift+Tab.
• all editing is painfully slow on Android (specifically, Firefox works well but slow).

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benweet/stackedit#25 has useful info.