Previously the communication between the sidebar and host frame was
implemented by a shared AnnotationSync class, with a 'crossframe'
service which abstracted the event bus on each side.

This design however assumed that both sides wanted to listen to
the same messages and react to them in similar ways. This is not
the case, especially given the change to use Redux for state
management in the sidebar app.

This commit replaces the crossframe service and AnnotationSync class
with a new 'frameSync' service which implements only the event
listeners and sidebar -> page RPC calls that are actually needed.

As a result, local annotation tags for annotations loaded via the API
can now be assigned by the reducer in the Redux store, making this
easier to test and getting us another step closer to making Annotation
objects immutable in the sidebar app.