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