(core) visualize simple differences between documents

Summary:
Render simple differences between documents.

 * Show cell changes.
 * Show cell conflicts.
 * Show row additions/deletions.

Doesn't support any schema changes, and is untested in the presence of schema changes.  Any widgets that access row data without using `cells` fields won't receive correct data.

Not addressed:
 * Rendering conflicts in mixed row addition/updating/deleting.
 * Column additions/deletions, option changes, etc.
 * Document level changes.
 * Table and column renames (though anticipated in ActionSummary structure).
 * Page-level changes.
 * Drawing attention to changes (marking changed pages+views, suppressing
   unchanged rows, etc).
 * Rendering differences in views other than GridView.
 * Adding UI for initiating a comparison.
 * Editing while comparing.

Replaces {D2600}

Test Plan: added tests

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2618

app/common/DocActions.ts

@@ -6,6 +6,12 @@
 import { CellValue } from 'app/plugin/GristData';
 export { CellValue, RowRecord } from 'app/plugin/GristData';
 
+// Part of a special CellValue used for comparisons, embedding several versions of a CellValue.
+export type CellVersions =
+  { parent: CellValue, remote: CellValue } |
+  { parent: CellValue, local: CellValue }  |
+  { parent: CellValue, local: CellValue, remote: CellValue };
+
 import map = require('lodash/map');
 
 export type AddRecord = ['AddRecord', string, number, ColValues];