Introduce a L10n-method to translate an element once, and use that in PDFLayerViewer

Currently we *manually* fetch the "pdfjs-additional-layers" string and update the DOM-element, which was needed since we want to avoid triggering a bunch of otherwise unnecessary translation when appending the entire layer-tree to the DOM.
By introducing a new helper method in the `L10n`-class we can avoid this, and instead use a "data-l10n-id" attribute on the element (as most other viewer code does nowadays).

web/l10n.js

@@ -80,6 +80,15 @@ class L10n {
     }
   }
 
+  /** @inheritdoc */
+  async translateOnce(element) {
+    try {
+      await this.#l10n.translateElements([element]);
+    } catch (ex) {
+      console.error(`translateOnce: "${ex}".`);
+    }
+  }
+
   /** @inheritdoc */
   async destroy() {
     for (const element of this.#elements) {