Turn integrations into event emitters

This will allow integrations to report events such as a change in the
loaded document or metadata.

src/annotator/integrations/html.js

+import { TinyEmitter } from 'tiny-emitter';
+
 import { anchor, describe } from '../anchoring/html';
 
 import { HTMLMetadata } from './html-metadata';
@@ -28,13 +30,15 @@ const MIN_HTML_WIDTH = 480;
  *
  * @implements {Integration}
  */
-export class HTMLIntegration {
+export class HTMLIntegration extends TinyEmitter {
   /**
    * @param {object} options
    *   @param {FeatureFlags} options.features
    *   @param {HTMLElement} [options.container]
    */
   constructor({ features, container = document.body }) {
+    super();
+
     this.features = features;
     this.container = container;
     this.anchor = anchor;

src/annotator/integrations/pdf.js

 import debounce from 'lodash.debounce';
 import { render } from 'preact';
+import { TinyEmitter } from 'tiny-emitter';
 
 import { ListenerCollection } from '../../shared/listener-collection';
 import {
@@ -62,7 +63,7 @@ export function isPDF() {
  * Integration that works with PDF.js
  * @implements {Integration}
  */
-export class PDFIntegration {
+export class PDFIntegration extends TinyEmitter {
   /**
    * @param {Annotator} annotator
    * @param {object} options
@@ -72,6 +73,8 @@ export class PDFIntegration {
    *     re-anchoring to complete when scrolling to an un-rendered page.
    */
   constructor(annotator, options = {}) {
+    super();
+
     this.annotator = annotator;
 
     const window_ = /** @type {HypothesisWindow} */ (window);

src/annotator/integrations/vitalsource.js

+import { TinyEmitter } from 'tiny-emitter';
+
 import { ListenerCollection } from '../../shared/listener-collection';
 import { FeatureFlags } from '../features';
 import { onDocumentReady } from '../frame-observer';
@@ -202,11 +204,13 @@ function makeContentFrameScrollable(frame) {
  *
  * @implements {Integration}
  */
-export class VitalSourceContentIntegration {
+export class VitalSourceContentIntegration extends TinyEmitter {
   /**
    * @param {HTMLElement} container
    */
   constructor(container = document.body) {
+    super();
+
     const features = new FeatureFlags();
 
     // Forcibly enable the side-by-side feature for VS books. This feature is

src/types/annotator.js

@@ -140,7 +140,7 @@
  *   This will only be called if the anchor has at least one highlight (ie.
  *   `anchor.highlights` is a non-empty array)
  *
- * @typedef {Destroyable & IntegrationBase} Integration
+ * @typedef {Destroyable & TinyEmitter & IntegrationBase} Integration
  */
 
 /**