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.

test/unit/ui_utils_spec.js

@@ -262,6 +262,45 @@ describe('ui_utils', function() {
       eventBus.dispatch('test');
       expect(count).toEqual(2);
     });
+
+    it('should not, by default, re-dispatch to DOM', function(done) {
+      if (isNodeJS()) {
+        pending('Document in not supported in Node.js.');
+      }
+      const eventBus = new EventBus();
+      let count = 0;
+      eventBus.on('test', function() {
+        count++;
+      });
+      document.addEventListener('test', function() {
+        count++;
+      });
+      eventBus.dispatch('test');
+
+      Promise.resolve().then(() => {
+        expect(count).toEqual(1);
+        done();
+      });
+    });
+    it('should re-dispatch to DOM', function(done) {
+      if (isNodeJS()) {
+        pending('Document in not supported in Node.js.');
+      }
+      const eventBus = new EventBus({ dispatchToDOM: true, });
+      let count = 0;
+      eventBus.on('test', function() {
+        count++;
+      });
+      document.addEventListener('test', function() {
+        count++;
+      });
+      eventBus.dispatch('test');
+
+      Promise.resolve().then(() => {
+        expect(count).toEqual(2);
+        done();
+      });
+    });
   });
 
   describe('isValidRotation', function() {