(core) Draft cells

Summary: Cells will remember their previous state when user pressed the escape key. Grist will offer a way to continue with the draft, by showing notification and a tooltip above the editor.

Test Plan: Browser tests were created

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2822

app/client/widgets/FieldBuilder.ts

@@ -492,6 +492,10 @@ export class FieldBuilder extends Disposable {
     // still maintain a Holder in this FieldBuilder is mainly to match older behavior; changing that
     // will entail a number of other tweaks related to the order of creating and disposal.
     this.gristDoc.fieldEditorHolder.autoDispose(fieldEditor);
+
+    // expose the active editor in a grist doc as an observable
+    fieldEditor.onDispose(() => this.gristDoc.activeEditor.set(null));
+    this.gristDoc.activeEditor.set(fieldEditor);
   }
 
   public isEditorActive() {