Add general support for re-dispatching events, on EventBus instances, to the DOM

This patch is the first step to be able to eventually get rid of the `attachDOMEventsToEventBus` function, by allowing `EventBus` instances to simply re-dispatch most[1] events to the DOM.
Note that the re-dispatching is purposely implemented to occur *after* all registered `EventBus` listeners have been serviced, to prevent the ordering issues that necessitated the duplicated page/scale-change events.

The DOM events are currently necessary for the `mozilla-central` tests, see https://hg.mozilla.org/mozilla-central/file/tip/browser/extensions/pdfjs/test, and perhaps also for custom deployments of the PDF.js default viewer.

Once this have landed, and been successfully uplifted to `mozilla-central`, I intent to submit a patch to update the test-code to utilize the new preference. This will thus, eventually, make it possible to remove the `attachDOMEventsToEventBus` functionality.

*Please note:* I've successfully ran all `mozilla-central` tests locally, with these patches applied.

---
[1] The exception being events that originated on the `window` or `document`, since those are already globally available anyway.

web/app.js

@@ -288,7 +288,8 @@ let PDFViewerApplication = {
     return new Promise((resolve, reject) => {
       this.overlayManager = new OverlayManager();
 
-      let eventBus = appConfig.eventBus || getGlobalEventBus();
+      const dispatchToDOM = AppOptions.get('eventBusDispatchToDOM');
+      let eventBus = appConfig.eventBus || getGlobalEventBus(dispatchToDOM);
       this.eventBus = eventBus;
 
       let pdfRenderingQueue = new PDFRenderingQueue();