(core) Move theme from ConfigNotifier to ThemeNotifier

Summary:
This reverts the behavior of onOptions, which had unintentionally
changed recently and no longer matched the API documentation.

Test Plan: Existing tests.

Reviewers: jarek

Reviewed By: jarek

Subscribers: paulfitz

Differential Revision: https://phab.getgrist.com/D4064

app/common/CustomWidget.ts

@@ -21,8 +21,8 @@ export interface ICustomWidget {
   /**
    * If set, Grist will render the widget after `grist.ready()`.
    *
-   * Currently, this is only used to defer rendering a widget until it has had
-   * a chance to apply the Grist theme.
+   * This is used to defer showing a widget on initial load until it has finished
+   * applying the Grist theme.
    */
   renderAfterReady?: boolean;
 }