Re-factor updating of thumbnails in the PDFSidebar-class

This patch does two things:
 - Moves the updating of thumbnails into `web/app.js`, via a new `PDFSidebar` callback-function, to avoid having to include otherwise unnecessary parameters when initializing a `PDFSidebar`-instance.
 - Only attempt to generate thumbnail-images from pages that are *cached* in the viewer. Note that only pages that exist in the `PDFPageViewBuffer`-instance can be rendered, hence it's not actually meaningful to check every single page when updating the thumbnails.
   For large documents, with thousands of pages, this should be a tiny bit more efficient when e.g. opening the sidebar since we no longer need to check pages that we know have not been rendered.

web/app.js

@@ -667,12 +667,23 @@ const PDFViewerApplication = {
     if (appConfig.sidebar) {
       this.pdfSidebar = new PDFSidebar({
         elements: appConfig.sidebar,
-        pdfViewer,
-        pdfThumbnailViewer: this.pdfThumbnailViewer,
         eventBus,
         l10n,
       });
       this.pdfSidebar.onToggled = this.forceRendering.bind(this);
+      this.pdfSidebar.onUpdateThumbnails = () => {
+        // Use the rendered pages to set the corresponding thumbnail images.
+        for (const pageView of pdfViewer.getCachedPageViews()) {
+          if (pageView.renderingState === RenderingStates.FINISHED) {
+            this.pdfThumbnailViewer
+              .getThumbnail(pageView.id - 1)
+              ?.setImage(pageView);
+          }
+        }
+        this.pdfThumbnailViewer.scrollThumbnailIntoView(
+          pdfViewer.currentPageNumber
+        );
+      };
     }
   },