(core) Cursor in custom widgets

Summary:
Adding a new method `setCursorPos` in the widget API, and a new configuration option for the ready message `allowSelectBy` that exposes custom widgets in the `Select by` dropdown.
With this, a custom widget can control the position of the linked widgets and is able to change the column in the creator panel.

Test Plan: Added new test. Existing tests should pass.

Reviewers: JakubSerafin

Reviewed By: JakubSerafin

Subscribers: JakubSerafin

Differential Revision: https://phab.getgrist.com/D3993

app/client/ui/selectBy.ts

@@ -127,7 +127,7 @@ function isValidLink(source: LinkNode, target: LinkNode) {
     }
 
     // custom widget must allow select by
-    if (!source.section.allowSelectBy.get()) {
+    if (!source.section.allowSelectBy()) {
       return false;
     }
   }