Notify `annotator` on individual annotation updates in `sidebar`

Currently, the sidebar (`frame-sync`) will notify the annotator to `loadAnnotations` (with a full set of annotations) and when an annotation is deleted (`deleteAnnotation`). We need something analogous for individual-annotation updates that might be relevant to the annotator (e.g. `updateAnnotation`).

The most immediate use case is for the new/prototype clustered highlights feature. There is a case in which a user might create a highlight, but later edit that highlight to add content: then it becomes an annotation. The annotator would need to know about that change so that it can update that annotation's clustering (it would move from `user-highlights` to `user-annotations` in this specific case).