The main `Guest` was assumed to be always in the same frame as the
`host` frame. In that scenario (which is currently the most common),
`Guest` was able to communicate with the `host` via `TinyEmitter`.

This PR removes that assumption an enforce all communication between the
`Guest` and the `host` to occur via an inter-frame communication
channel. This paves the work for the main `Guest` to be in a different
frame than the `host`, which is the case in the Ebook readers and
VitalSource.