[api-minor] Modernize and simplify the ProgressBar class

The original `ProgressBar`-functionality is very old, and could thus do with some general clean-up.
In particular, while it currently accepts various options those have never really been used in either the default viewer or in any examples. The sort of "styling" that these options provided are *much better*, not to mention simpler, done directly with CSS rules.

As part of these changes, the "progress" is now updated using CSS variables rather than by directly modifying the `style` of DOM elements. This should hopefully simplify future changes to this code, see e.g. PR 14898.

Finally, this also fixes a couple of other small things in the "mobile viewer" example.

examples/mobile-viewer/viewer.css

@@ -13,6 +13,10 @@
  * limitations under the License.
  */
 
+:root {
+  --progressBar-percent: 0%;
+}
+
 * {
   padding: 0;
   margin: 0;
@@ -191,13 +195,12 @@ canvas {
   height: 0.6rem;
   background-color: rgba(51, 51, 51, 1);
   border-bottom: 1px solid rgba(51, 51, 51, 1);
-  margin-top: 5rem;
 }
 
 #loadingBar .progress {
   position: absolute;
   left: 0;
-  width: 0;
+  width: var(--progressBar-percent);
   height: 100%;
   background-color: rgba(221, 221, 221, 1);
   overflow: hidden;
@@ -217,6 +220,7 @@ canvas {
 }
 
 #loadingBar .progress.indeterminate {
+  width: 100%;
   background-color: rgba(153, 153, 153, 1);
   transition: none;
 }

examples/mobile-viewer/viewer.js

@@ -82,7 +82,9 @@ const PDFViewerApplication = {
         self.pdfDocument = pdfDocument;
         self.pdfViewer.setDocument(pdfDocument);
         self.pdfLinkService.setDocument(pdfDocument);
-        self.pdfHistory.initialize({ fingerprint: pdfDocument.fingerprint });
+        self.pdfHistory.initialize({
+          fingerprint: pdfDocument.fingerprints[0],
+        });
 
         self.loadingBar.hide();
         self.setTitleUsingMetadata(pdfDocument);
@@ -159,7 +161,7 @@ const PDFViewerApplication = {
   },
 
   get loadingBar() {
-    const bar = new pdfjsViewer.ProgressBar("#loadingBar", {});
+    const bar = new pdfjsViewer.ProgressBar("#loadingBar");
 
     return pdfjsLib.shadow(this, "loadingBar", bar);
   },
@@ -187,7 +189,7 @@ const PDFViewerApplication = {
       // Provides some basic debug information
       console.log(
         "PDF " +
-          pdfDocument.fingerprint +
+          pdfDocument.fingerprints[0] +
           " [" +
           info.PDFFormatVersion +
           " " +