This is a step towards making the bucket bar works for a `guest` frame
that is not in the `host` frame. When that's the case the observation of
scrolling and resize can't happen in the `host` frame.

A new service has been created, `BucketService`. The service is
instantiated only in the `guest` frame that contains the main
annotatable content. The service observes changes in the elements that
could impact the relative position of the anchors relative to the
viewport. If these changes are detected, as well as addition or deletion
of anchors, it sends the `anchorsChanged` RPC event to the `host` frame
so that the positions can be recalculated. As a result of the new
service the `BucketBar` has been simplified.