Introduce simpler rebasing.

This is an attempt at a much simpler rebasing approach. This should have
several advantages over the HTMLParser based approach, for example:

- With the HTMLParser approach tag soup interpretation is done in js.
  This can often result in a DOM representation that differs from the
  one the underlying browser would create on its own. This can result in
  pages looking differently if viewed directly or if injected by another
  page.

- HTMLParser is DOCTYPE agnostic. If the browser's innerHTML routine
  does respect DOCTYPE it can create more precise DOM representations.

The old rebaser is still in the code, so we can investigate the
differences between both approaches.