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_options.js

@@ -68,6 +68,11 @@ const defaultOptions = {
     value: false,
     kind: OptionKind.VIEWER,
   },
+  eventBusDispatchToDOM: {
+    /** @type {boolean} */
+    value: false,
+    kind: OptionKind.VIEWER,
+  },
   externalLinkRel: {
     /** @type {string} */
     value: 'noopener noreferrer nofollow',