AnnotationSync utility didn't have a clear purpose, except for assigning a
temporarily `$tag` to newly created annotations. For that, it used an
internal cache of annotation objects from the Guest, that were mutated
in-place. `AnnotationSync` used a very indirect method of communication
to the `Guest`, using an event emitter, even though their mutual
relationship was quite straightforward.

In this PR, we have:

* Simplified the messages that are sent between the host and the
  sidebar. We have assumed that a `$tag` value is always present if it
  comes from the `sidebar`, and a temporarily `$tag` is assigned in the
  `Guest` class for newly created annotations. Therefore, the type of
  the message sent between these two frames can be simplified to always
  be of the shape of `AnnotationData`, a safe version of `Annotation`.

* Added a few missing types.

* Changed the RPC method `deleteAnnotation` to send astring tag, similar
  to other RPC methods, like `focusAnnotations` or `scrollToAnnoation`.
  As a consequence of that we changed the `Guest#detach` method
  signature.