🐛 [#3921] Expose all components instead of just within the same parent

For nested components in a layout component, we need to be able to
select components that are outside of this container too, so instead
of only considering the parent (which is the root for non-nested
fields), look at the root container holding all the form components.

Backport-of: #3949

src/openforms/js/components/formio_builder/WebformBuilder.js

@@ -385,7 +385,7 @@ class WebformBuilder extends WebformBuilderFormio {
             uniquifyKey={uniquifyKey}
             supportedLanguageCodes={LANGUAGES}
             richTextColors={RICH_TEXT_COLORS}
-            getFormComponents={() => parent.formioContainer}
+            getFormComponents={() => this.webform.components}
             getValidatorPlugins={getValidatorPlugins}
             getRegistrationAttributes={getRegistrationAttributes}
             getPrefillPlugins={getPrefillPlugins}