Add online help for keyboard shortcuts

First step towards #7: add online help, tooltips
Fix the way SVG nodes are created

Using document.createElementNS now instead of having browser reparse
HTML.
s/let/var/

Since let isn't quite supported by default in the major browsers.

Fix #3: make compatible with Firefox and default Chrome
Fix SVG text vertical alignment

Wasn't standards-compliant; text was too high on Firefox.
Use recursive setTimeout rather than setInterval

MDN suggests this if there's any risk that the logic could take longer
than 10ms:
https://developer.mozilla.org/en-US/docs/Web/API/window.setInterval .
However, graphics still slow down when there's too much going on in
Firefox, so this is only part of the solution.
Make update interval a constant
Slightly optimize redrawing servers
Switch from setTimeout to requestAnimationFrame

Things are smoother now in Firefox, no more slowdown when there's too
much going on.
Add some tooltips

Close #7: add online help, tooltips
Group logs in SVG
Lint