Bidirectional Linking (#622)

Allows bidirectional / cyclic linking for same-record cursor links.
This should allow multiple sections to all synchronize their cursors,
such that clicking in any one of them will move all the others.

Works even if some sections in the cycle have rows filtered out (the 
filtered-out sections might desync their cursors, but the correct cursor
position will still propagate downstream, and they'll re-sync if clicking on
a row that is present in them)

Under the hood, each cursor has a _lastEditedAt counter, updated when
a user's action changes the cursor in a section, such that we can always
tell which section was touched most recently. This is used to resolve
conflicts stably when dealing with cycles or chains of cursor-links.

Updated selectBy and recursiveMoveToCursorPos to handle cycles

Updated tests for selectBy behavior

However, main bidirectional-linking tests are not in this commit, they'll come in a subsequent PR

app/client/models/entities/ViewSectionRec.ts

@@ -1,4 +1,5 @@
 import BaseView from 'app/client/components/BaseView';
+import {SequenceNEVER, SequenceNum} from 'app/client/components/Cursor';
 import {EmptyFilterColValues, LinkingState} from 'app/client/components/LinkingState';
 import {KoArray} from 'app/client/lib/koArray';
 import {ColumnToMapImpl} from 'app/client/models/ColumnToMap';
@@ -155,6 +156,8 @@ export interface ViewSectionRec extends IRowModel<"_grist_Views_section">, RuleO
 
   activeRowId: ko.Observable<UIRowId | null>;     // May be null when there are no rows.
 
+  lastCursorEdit: ko.Observable<SequenceNum>;
+
   // If the view instance for section is instantiated, it will be accessible here.
   viewInstance: ko.Observable<BaseView | null>;
 
@@ -627,6 +630,7 @@ export function createViewSectionRec(this: ViewSectionRec, docModel: DocModel):
   this.linkTargetCol = refRecord(docModel.columns, this.linkTargetColRef);
 
   this.activeRowId = ko.observable<UIRowId|null>(null);
+  this.lastCursorEdit = ko.observable<SequenceNum>(SequenceNEVER);
 
   this._linkingState = Holder.create(this);
   this.linkingState = this.autoDispose(ko.pureComputed(() => {