Remove jQuery dependency from BucketBar

Remove jQuery dependency from BucketBar, its tests
and other relevant modules' tests involving BucketBar.

Opportunistically add some type-checking, commenting and
other small improvements but leave general structure as-is.

Increase test coverage.

src/annotator/sidebar.js

@@ -42,7 +42,9 @@ export default class Sidebar extends Host {
     }
 
     if (this.plugins.BucketBar) {
-      this.plugins.BucketBar.element.on('click', () => this.show());
+      this.plugins.BucketBar.element.addEventListener('click', () =>
+        this.show()
+      );
     }
 
     // Set up the toolbar on the left edge of the sidebar.

src/annotator/test/pdf-sidebar-test.js

-import $ from 'jquery';
-
 import PdfSidebar from '../pdf-sidebar';
 import { $imports } from '../pdf-sidebar';
 
@@ -46,7 +44,7 @@ describe('PdfSidebar', () => {
     fakeCrossFrame.destroy = sandbox.stub();
 
     const fakeBucketBar = {};
-    fakeBucketBar.element = $('<div></div>');
+    fakeBucketBar.element = document.createElement('div');
     fakeBucketBar.destroy = sandbox.stub();
 
     CrossFrame = sandbox.stub();

src/annotator/test/sidebar-test.js

-import $ from 'jquery';
-
 import events from '../../shared/bridge-events';
 
 import Sidebar from '../sidebar';
@@ -68,7 +66,7 @@ describe('Sidebar', () => {
     FakeToolbarController = sinon.stub().returns(fakeToolbar);
 
     const fakeBucketBar = {};
-    fakeBucketBar.element = $('<div></div>');
+    fakeBucketBar.element = document.createElement('div');
     fakeBucketBar.destroy = sandbox.stub();
 
     CrossFrame = sandbox.stub();