(core) Adds setSelectedRows to the grist api for custom view

Summary:
This is needed to let custom widget driver filtering of other widget in the same page.

Descripion here:
 - https://grist.quip.com/ctytAQJoFMsM/Hopefully-Small-Projects#temp:C:NNCfe2030b27647439886ca83595

Test Plan: New api tested in a new nbrowser test

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D3253

app/client/ui/selectBy.ts

@@ -74,11 +74,24 @@ function isValidLink(source: LinkNode, target: LinkNode) {
     return false;
   }
 
-  // cannot select from chart or custom
-  if (['chart', 'custom'].includes(source.widgetType)) {
+  // cannot select from chart
+  if (source.widgetType === 'chart') {
     return false;
   }
 
+  if (source.widgetType === 'custom') {
+
+    // custom widget do not support linking by columns
+    if (source.tableId !== source.section.table.peek().primaryTableId.peek()) {
+      return false;
+    }
+
+    // custom widget must allow select by
+    if (!source.section.allowSelectBy.get()) {
+      return false;
+    }
+  }
+
   // The link must not create a cycle
   if (source.ancestors.has(target.section.getRowId())) {
     return false;