After #4176, the 'create note' button has stopped working in
VitalSource. That's because we don't create the guest frame in the host
frame. Creating a note relayed on one of the guest frames having a
frameIdentifier` with value `null`. That's not longer the case.

Now, when creating a page note we send the request to the first
connected guest frame. This is analogous to #4177.

In addition, we only send `textUnselected`, `textSelected` and
`createAnnotation` RPC events to only specific guest frames. That
simplifies the logic by avoiding to send a `frameIdentifier`.