[api-minor] Add support for toggling of Optional Content in the viewer (issue 12096)

*Besides, obviously, adding viewer support:* This patch attempts to improve the general API for Optional Content Groups slightly, by adding a couple of new methods for interacting with the (more complex) data structures of `OptionalContentConfig`-instances. (Thus allowing us to mark some of the data as "private", given that it probably shouldn't be manipulated directly.)

By utilizing not just the "raw" Optional Content Groups, but the data from the `/Order` array when available, we can thus display the Layers in a proper tree-structure with collapsible headings for PDF documents that utilizes that feature.

Note that it's possible to reset all Optional Content Groups to their default visibility state, simply by double-clicking on the Layers-button in the sidebar.
(Currently that's indicated in the Layers-button tooltip, which is obviously easy to overlook, however it's probably the best we can do for now without adding more buttons, or even a dropdown-toolbar, to the sidebar.)

Also, the current Layers-button icons are a little rough around the edges, quite literally, but given that the viewer will soon have its UI modernized anyway they hopefully suffice in the meantime.

To give users *full* control of the visibility of the various Optional Content Groups, even those which according to the `/Order` array should not (by default) be toggleable in the UI, this patch will place those under a *custom* heading which:
 - Is collapsed by default, and placed at the bottom of the Layers-tree, to be a bit less obtrusive.
 - Uses a slightly different formatting, compared to the "regular" headings.
 - Is localizable.

Finally, note that the thumbnails are *purposely* always rendered with all Optional Content Groups at their default visibility state, since that seems the most useful and it's also consistent with other viewers.
To ensure that this works as intended, we'll thus disable the `PDFThumbnailView.setImage` functionality when the Optional Content Groups have been changed in the viewer. (This obviously means that we'll re-render thumbnails instead of using the rendered pages. However, this situation ought to be rare enough for this to not really be a problem.)

web/base_viewer.js

@@ -439,6 +439,7 @@ class BaseViewer {
     const firstPagePromise = pdfDocument.getPage(1);
 
     const annotationStorage = pdfDocument.annotationStorage;
+    const optionalContentConfigPromise = pdfDocument.getOptionalContentConfig();
 
     this._pagesCapability.promise.then(() => {
       this.eventBus.dispatch("pagesloaded", {
@@ -474,6 +475,7 @@ class BaseViewer {
     firstPagePromise
       .then(firstPdfPage => {
         this._firstPageCapability.resolve(firstPdfPage);
+        this._optionalContentConfigPromise = optionalContentConfigPromise;
 
         const scale = this.currentScale;
         const viewport = firstPdfPage.getViewport({ scale: scale * CSS_UNITS });
@@ -486,8 +488,9 @@ class BaseViewer {
             eventBus: this.eventBus,
             id: pageNum,
             scale,
-            annotationStorage,
             defaultViewport: viewport.clone(),
+            annotationStorage,
+            optionalContentConfigPromise,
             renderingQueue: this.renderingQueue,
             textLayerFactory,
             textLayerMode: this.textLayerMode,
@@ -605,6 +608,7 @@ class BaseViewer {
     this._buffer = new PDFPageViewBuffer(DEFAULT_CACHE_SIZE);
     this._location = null;
     this._pagesRotation = 0;
+    this._optionalContentConfigPromise = null;
     this._pagesRequests = new WeakMap();
     this._firstPageCapability = createPromiseCapability();
     this._onePageRenderedCapability = createPromiseCapability();
@@ -1222,6 +1226,50 @@ class BaseViewer {
     });
   }
 
+  /**
+   * @type {Promise<OptionalContentConfig | null>}
+   */
+  get optionalContentConfigPromise() {
+    if (!this.pdfDocument) {
+      return Promise.resolve(null);
+    }
+    if (!this._optionalContentConfigPromise) {
+      // Prevent issues if the getter is accessed *before* the `onePageRendered`
+      // promise has resolved; won't (normally) happen in the default viewer.
+      return this.pdfDocument.getOptionalContentConfig();
+    }
+    return this._optionalContentConfigPromise;
+  }
+
+  /**
+   * @param {Promise<OptionalContentConfig>} promise - A promise that is
+   *   resolved with an {@link OptionalContentConfig} instance.
+   */
+  set optionalContentConfigPromise(promise) {
+    if (!(promise instanceof Promise)) {
+      throw new Error(`Invalid optionalContentConfigPromise: ${promise}`);
+    }
+    if (!this.pdfDocument) {
+      return;
+    }
+    if (!this._optionalContentConfigPromise) {
+      // Ignore the setter *before* the `onePageRendered` promise has resolved,
+      // since it'll be overwritten anyway; won't happen in the default viewer.
+      return;
+    }
+    this._optionalContentConfigPromise = promise;
+
+    for (const pageView of this._pages) {
+      pageView.update(pageView.scale, pageView.rotation, promise);
+    }
+    this.update();
+
+    this.eventBus.dispatch("optionalcontentconfigchanged", {
+      source: this,
+      promise,
+    });
+  }
+
   /**
    * @type {number} One of the values in {ScrollMode}.
    */