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/pdf_viewer.js

@@ -341,6 +341,10 @@ class PDFViewer {
     return this._pages[index];
   }
 
+  getCachedPageViews() {
+    return new Set(this.#buffer);
+  }
+
   /**
    * @type {boolean} - True if all {PDFPageView} objects are initialized.
    */