In order to clean up vocabulary and better serve the needs of dynamic
pages, refactor `setupAnnotation` to deal with re-setup, unhighlighting
targets that have been removed from the annotation and re-anchoring
anchors that have lost their ranges.

- Store the Range on the anchor

  * The BucketBar would use it for getBoundingClientRect, except that
    it's a bit broken on Chrome.

  * The PDF Plugin can delete it to trigger re-anchoring no page load

- Use MutationObservers in the PDF plugin and make smarter judgements
  about when pages are done rendering by using the renderState in
  addition to the text layer renderingDone flag.

Sometimes the context might hinder the anchoring if the context has
changed but the quote has not.

TextPositionAnchor can be generated and re-anchored within a PDF so
long as the selection doesn't span a page boundary.

TextQuoteAnchor is generated, but not re-anchored.

Rather than having each of the anchoring implementations have different
call signatures for their methods, use an options argument that varies
in content between the implementations, but probably shares some core
options like the root node.

The core of anchoring in the guest code is now asynchronous.

- Anchoring annotations happens asynchronously in the setup function.

- Creating an annotation is asynchronous, and depends upon the resolution
  of the targets, which may depend on the resolution of the document metadata.
  The first events fire only after this happens.

- The Anchor API can now optionally return Promises when converting ranges
  and selectors, though all the present implementations are synchronous.

It went down something like this:

:city_sunset::sparkles::alien::sweat_drops::pray::boom::city_sunrise:

s/viewable/visible/