Bug 16735: about:tor should accommodate different fonts/font sizes.

Avoid hard-coded heights and positions in about:tor. The "update needed"
  arrow is now constructed from 4 elements instead of 2 and the info.
  "bubbles" use table layout instead of hard-coded heights.
Set the CSS z-index so that text will be in front of the status image
  (onion image) and the "update needed" arrow.
Use "let" instead of "var" in the about:tor JS code.