We want the `BucketBar` to be able to communicate with `Guest`s that are
in other iframes, like in the Ebook scenario. Currently, the `BucketBar`
has an instance of the `Guest`, associated with `host` frame, and
assumes to be the main `Guest`, for communication purposes.

I have replaced the reliance of the direct invocation of
`Guest#selectAnnotations` by a new `selectAnnotations` RPC event in the
`guest-host` communication channel. This has the advantage of making no
assumptions about which iframe contains the annotations.

I have made some minor modification in the types and functionality of
`range-util`.