Extract common functionality into a new BaseTreeViewer._finishRendering method

Note how the end of the `{PDFOutlineViewer, PDFAttachmentViewer, PDFLayerViewer}.render` methods share *almost* identical code, hence we can reduce some duplication by introducing the new `BaseTreeViewer` helper method here.

Furthermore, setting `this._lastToggleIsShow` can be made ever so slightly more efficient, since we don't care about the number of ".treeItemsHidden"-classes but only want to know if at least one exists.

web/base_tree_viewer.js

@@ -103,6 +103,20 @@ class BaseTreeViewer {
     this._toggleTreeItem(this.container, !this._lastToggleIsShow);
   }
 
+  /**
+   * @private
+   */
+  _finishRendering(fragment, count, hasAnyNesting = false) {
+    if (hasAnyNesting) {
+      this.container.classList.add("treeWithDeepNesting");
+
+      this._lastToggleIsShow = !fragment.querySelector(".treeItemsHidden");
+    }
+    this.container.appendChild(fragment);
+
+    this._dispatchEvent(count);
+  }
+
   render(params) {
     throw new Error("Not implemented: render");
   }