f96af43d added support for PDF.js >= v2.5.207 by changing an event
listener to use PDF.js's internal event bus rather than DOM events.
However there was another DOM event listener for the `pagesloaded` event
in `src/annotator/anchoring/pdf.js` that should also have been updated but was
overlooked. This didn't cause a problem in testing with the dev server because
the test documents load quickly enough that they are already loaded by the time
the client's anchoring logic ran.

This commit updates the way that the client listens for events from
PDF.js to use the event bus where available and only fall back to the
DOM in versions of PDF.js that don't support it.

 - Use PDF.js's event bus to listen for `documentload`/`documentloaded`
   and `pagesloaded` events

 - Add a fallback method to wait for event bus to become available in
   versions of PDF.js which support the eventBus but don't have the
   `initializedPromise` API

 - Improve the documentation around which versions of PDF.js support
   different event types and event dispatch methods

 - Add tests to cover the behavior from different releases of PDF.js

For an overview of the different versions of PDF.js that the client
needs to support, see https://github.com/hypothesis/client/issues/2643.