(core) custom widget appear as build-in widget

Summary: Added boilerplate code needed to create new wigets in "Add new" menu, that are wrapped around existing custom widgets. More details can be found here: https://grist.quip.com/larhAGRKyl6Z/Custom-widgets-in-Add-Widget-menu

Test Plan: nbowser tests added to verify if item in menu exits, if widget is rendered, and right side menu has widget selection and read access selection hided.

Reviewers: georgegevoian

Reviewed By: georgegevoian

Differential Revision: https://phab.getgrist.com/D3994

app/client/ui/PredefinedCustomSectionConfig.ts

@@ -0,0 +1,23 @@
+import {GristDoc} from "../components/GristDoc";
+import {ViewSectionRec} from "../models/entities/ViewSectionRec";
+import {CustomSectionConfig} from "./CustomSectionConfig";
+
+export class PredefinedCustomSectionConfig extends CustomSectionConfig {
+
+
+  constructor(section: ViewSectionRec, gristDoc: GristDoc) {
+    super(section, gristDoc);
+  }
+
+  public buildDom() {
+    return this._customSectionConfigurationConfig.buildDom();
+  }
+
+  protected shouldRenderWidgetSelector(): boolean {
+    return false;
+  }
+
+  protected async _getWidgets(): Promise<void> {
+    // Do nothing.
+  }
+}