In order to toggle correctly the `Toolbar` button between `New page
note` and `New annotation` we have created a new `guest-host` channel of
communication. Previous code failed to toggle the button because it used
an event emitter, hence assuming that the `Guest` was in the same frame
as the host.

- The `Sidebar` keeps track of the iframe that has currently text
  selected. It uses the `subFrameIdentifier`. If there is no
  `subFrameIdentifier` (currently the top `Guest` frame), it sets the
  identifier to 'main', referring to the frame with the main annotatable
  content. This will change in a follow up PR that will add more logic
  into which frame is tagged as 'main'.

- Only one iframe at a time can have selected text.

- The event emitter `hasSelectionChanged` has been replaced by three RPC
  methods: 'textSelectedAt`, 'textDeselectedAt', 'deselecTextExcept'

- `deselecTextExcept` is used to clear text selections in other frames,
  except from the iframe that has the latest selection. This is not the
  neatest solution, but for now it works.

On a follow up PR I will address the removal of the shadow left from
previous a selection.

Closes https://github.com/hypothesis/product-backlog/issues/1236