(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/plugin/CustomSectionAPI.ts

@@ -50,6 +50,10 @@ export interface InteractionOptionsRequest {
    * and those requested by Custom Widget.
    */
   columns?: ColumnsToMap,
+  /**
+   * Show widget as linking source.
+   */
+  allowSelectBy?: boolean,
 }
 
 /**