(core) Adds setSelectedRows to the grist api for custom view

Summary:
This is needed to let custom widget driver filtering of other widget in the same page.

Descripion here:
 - https://grist.quip.com/ctytAQJoFMsM/Hopefully-Small-Projects#temp:C:NNCfe2030b27647439886ca83595

Test Plan: New api tested in a new nbrowser test

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D3253

app/client/models/entities/ViewSectionRec.ts

@@ -22,7 +22,7 @@ import {arrayRepeat} from 'app/common/gutil';
 import {Sort} from 'app/common/SortSpec';
 import {ColumnsToMap, WidgetColumnMap} from 'app/plugin/CustomSectionAPI';
 import {ColumnToMapImpl} from 'app/client/models/ColumnToMap';
-import {Computed} from 'grainjs';
+import {Computed, Observable} from 'grainjs';
 import * as ko from 'knockout';
 import defaults = require('lodash/defaults');
 
@@ -159,6 +159,12 @@ export interface ViewSectionRec extends IRowModel<"_grist_Views_section"> {
   // Temporary variable holding widget desired access (changed either from manifest or via API).
   desiredAccessLevel: ko.Observable<AccessLevel|null>;
 
+  // Show widget as linking source. Used by custom widget.
+  allowSelectBy: Observable<boolean>;
+
+  // List of selected rows
+  selectedRows: Observable<number[]>;
+
   // Save all filters of fields/columns in the section.
   saveFilters(): Promise<void>;
 
@@ -562,4 +568,7 @@ export function createViewSectionRec(this: ViewSectionRec, docModel: DocModel):
     }
     return result;
   });
+
+  this.allowSelectBy = Observable.create(this, false);
+  this.selectedRows = Observable.create(this, []);
 }