(core) Adding onNewRecord event for custom widgets

Summary:
New onNewRecord event in custom widgets that is triggered
when the user selects a blank row.

Together with this diff there is PR for grist-help https://github.com/gristlabs/grist-help/pull/150

Test Plan: added test case

Reviewers: cyprien

Reviewed By: cyprien

Differential Revision: https://phab.getgrist.com/D3375

app/client/components/CustomView.ts

@@ -73,7 +73,7 @@ export class CustomView extends Disposable {
   private _emptyWidgetPage: string;
 
   public create(gristDoc: GristDoc, viewSectionModel: ViewSectionRec) {
-    BaseView.call(this as any, gristDoc, viewSectionModel);
+    BaseView.call(this as any, gristDoc, viewSectionModel, { 'addNewRow': true });
 
     this._customDef =  this.viewSection.customDef;
 

app/client/components/WidgetFrame.ts

@@ -328,7 +328,7 @@ export class GristViewImpl implements GristView {
     // Hidden/Visible columns will eventually reflect what is available, but this operation
     // is not instant - and widget can receive rows with fields that are not in the mapping.
     const columns: ColumnRec[] = this._visibleColumns();
-    const rowIds: number[] = this._baseView.sortedRows.getKoArray().peek() as number[];
+    const rowIds = this._baseView.sortedRows.getKoArray().peek().filter(id => id != 'new');
     const data: BulkColValues = {};
     for (const column of columns) {
       // Use the colId of the displayCol, which may be different in case of Reference columns.