The only way to get information about the location of the annotations
in the document is to request this information from the highlights.

This information can't be read out from the selectors, because anchoring
can place them *anywhere* in the document.

They also can't be read from the anchors, because anchors can contain
any kind of description; there is no mapping to any uniform coordinate
system.

The only way to get this information is to ask the hights for
the coordinates, after they have been rendered.

So this must be done after the second-phase of anchoring; after the
rendering of the highlights in the DOM.

So now we query this info from the highlights (upon rendering
in the host fram), and then store it back into the targets,
which are then sent back to the sidebar, so that this information
is available for sorting.

Fixes #1217.