When using a custom bucket container, the top / bottom buckets may be positioned
differently than the code previously assumed (137x px below the top of the
viewport, 20px above the bottom). Redo the logic to work regardless of bucket
container positioning. Instead of trying to determine whether an anchor is
on-screen or not, just pick the anchor with the lowest bottom or highest top as
the "nearest" one, depending on which direction we are scrolling in.

This is implemented by:

 - Changing `Bucket` objects to record not just the tags from anchors in
   them, but the list of `AnchorPosition` objects from which the bucket
   was built. These objects include the top and bottom positions of the
   anchor.

 - Make clicking on the "Up" bucket search the anchor list for the
   anchor with the lowest bottom position, and send a request to the
   guest to scroll to that anchor.

 - Make clicking on the "Down" bucket search the anchor list for the
   anchor with the highest top position, and send a request to the
   guest to scroll to that anchor.

 - Removing all the logic for finding and scrolling to the "nearest
   off-screen anchor"