(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-ti.ts

@@ -19,6 +19,7 @@ export const InteractionOptionsRequest = t.iface([], {
   "requiredAccess": t.opt("string"),
   "hasCustomOptions": t.opt("boolean"),
   "columns": t.opt("ColumnsToMap"),
+  "allowSelectBy": t.opt("boolean"),
 });
 
 export const InteractionOptions = t.iface([], {