Make setupAnnotation into a tiny compatibility wrapper around a new
method #anchor() method that returns a promise of the anchors. This
is cleaner than trying to store the anchors on the annotation itself,
which creates circular references, and it means there's no need to
strip these properties when serializes annotations across the frame
boundary.

Sort annotations by TextPositionSelector in the widget display. This
provides greater visual stability -- cards don't change positions
after anchoring -- at the cost of potentially out of order cards when
anchors have moved.