Change the logic for making highlights visible when creating a new
annotation, so that it works if the annotation was created in a non-host
frame.

This is done by moving the logic from the `beforeAnnotationCreated`
handler in the Sidebar class, which was only called for annotations in
the host frame, to the `beforeCreateAnnotation` handler in
FrameSyncService, which is called for all frames. The sidebar app will
then send a request to show highlights to the host frame, which will
update the sidebar's controls and then relay the request to guest
frames.