(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/components/WidgetFrame.ts

@@ -360,6 +360,14 @@ export class GristViewImpl implements GristView {
     return data;
   }
 
+  public async allowSelectBy(): Promise<void> {
+    this._baseView.viewSection.allowSelectBy.set(true);
+  }
+
+  public async setSelectedRows(rowIds: number[]): Promise<void> {
+    this._baseView.viewSection.selectedRows.set(rowIds);
+  }
+
   private _visibleColumns() {
     const columns: ColumnRec[] = this._baseView.viewSection.columns.peek();
     const hiddenCols = this._baseView.viewSection.hiddenColumns.peek().map(c => c.id.peek());