(core) Adding backend for 2-way references

Summary:
Adding support for 2-way references in data engine.

- Columns have an `reverseCol` field, which says "this is a reverse of the given column, update me when that one changes".
- At the time of setting `reverseCol`, we ensure that it's symmetrical to make a 2-way reference.
- Elsewhere we just implement syncing in one direction:
  - When `reverseCol` is present, user code is generated with a type like `grist.ReferenceList("Tasks", reverse_of="Assignee")`
  - On updating a ref column, we use `prepare_new_values()` method to generate corresponding updates to any column that's a reverse of it.
  - The `prepare_new_values()` approach is extended to support this.
  - We don't add (or remove) any mappings between rows, and rely on existing mappings (in a ref column's `_relation`) to create reverse updates.

NOTE This is polished version of https://phab.getgrist.com/D4307 with tests and 3 bug fixes
- Column transformation didn't work when transforming RefList to Ref, the reverse column became out of sync
- Tables with reverse columns couldn't be removed
- Setting json arrays to RefList didn't work if arrays contained other things besides ints
Those fixes are covered by new tests.

Test Plan: New tests

Reviewers: georgegevoian, paulfitz, dsagal

Reviewed By: georgegevoian, paulfitz

Subscribers: dsagal

Differential Revision: https://phab.getgrist.com/D4322

app/client/components/DataTables.ts

@@ -12,8 +12,9 @@ import {icon} from 'app/client/ui2018/icons';
 import {loadingDots} from 'app/client/ui2018/loaders';
 import {menu, menuDivider, menuIcon, menuItem, menuItemAsync, menuText} from 'app/client/ui2018/menus';
 import {confirmModal} from 'app/client/ui2018/modals';
-import {Computed, Disposable, dom, fromKo, makeTestId, observable, Observable, styled} from 'grainjs';
+import {Computed, Disposable, dom, fromKo, observable, Observable, styled} from 'grainjs';
 import {makeT} from 'app/client/lib/localization';
+import {makeTestId} from 'app/client/lib/domUtils';
 import * as weasel from 'popweasel';
 
 const testId = makeTestId('test-raw-data-');
@@ -109,6 +110,7 @@ export class DataTables extends Disposable {
                 ),
                 cssDotsButton(
                   testId('table-menu'),
+                  testId(use => `table-menu-${use(tableRec.tableId)}`),
                   icon('Dots'),
                   menu(() => this._menuItems(tableRec, isEditingName), {placement: 'bottom-start'}),
                   dom.on('click', (ev) => { ev.stopPropagation(); ev.preventDefault(); }),