Change how the sidebar is notified of guest frames being unloaded to
support guest frames where the client has been loaded via means other
than `HypothesisInjector` or where the guest is cross-origin.

Instead of listening for the guest frame's 'unload' event from the
parent frame in `HypothesisInjector`, the guest frame instead listens
for this event itself and sends a `hypothesisGuestUnloaded` message to
the host frame via `window.postMessage`, which in turn is handled in the
`Sidebar` class to relay it to the sidebar app via a `destroyFrame` RPC
call. This indirect route works around a bug in Safari (see code
comments).

As well as supporting future use cases, this also simplifies the
`HypothesisInjector` class as it no longer needs access to the `Bridge`.