Issue #3470670 by mably: Add a custom selector option for the Ajax MutationObserver target

Closes #3470670

config/schema/toc_js.schema.yml

@@ -83,6 +83,9 @@ node.type.*.third_party.toc_js:
     ajax_page_updates:
       type: integer
       label: 'Handle Ajax page updates'
+    observable_selector:
+      type: string
+      label: 'Selector for Ajax MutationObserver target'
 
 block.settings.toc_js_block:
   type: block_settings
@@ -166,3 +169,6 @@ block.settings.toc_js_block:
     ajax_page_updates:
       type: integer
       label: 'Handle Ajax page updates'
+    observable_selector:
+      type: string
+      label: 'Selector for Ajax MutationObserver target'