(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/components/CustomCalendarView.ts

@@ -0,0 +1,31 @@
+import {AccessLevel} from "app/common/CustomWidget";
+import {ViewSectionRec} from "app/client/models/entities/ViewSectionRec";
+import {CustomView} from "app/client/components/CustomView";
+import {GristDoc} from "app/client/components/GristDoc";
+
+//Abstract class for more future inheritances
+abstract class CustomAttachedView extends CustomView {
+  public override create(gristDoc: GristDoc, viewSectionModel: ViewSectionRec) {
+    super.create(gristDoc, viewSectionModel);
+    void viewSectionModel.customDef.access.setAndSave(AccessLevel.full);
+
+    const widgetsApi = this.gristDoc.app.topAppModel.api;
+    widgetsApi.getWidgets().then(async result=>{
+      const widget = result.find(w=>w.name == this.getWidgetName());
+      if(widget) {
+        await this.customDef.url.setAndSave(widget.url);
+      }
+    }).catch(()=>{
+      //do nothing
+    });
+  }
+
+  protected abstract getWidgetName(): string;
+
+}
+
+export class CustomCalendarView extends CustomAttachedView {
+  protected getWidgetName(): string {
+    return "Calendar";
+  }
+}