First draft of hierarchical widget.

Closes #3491436

config/schema/custom_field.schema.yml

@@ -505,6 +505,12 @@ custom_field.widget.*:
                 global_types:
                   type: boolean
                   label: 'Global types'
+            force_deepest_level:
+              type: boolean
+              label: 'Force deepest level'
+            level_labels:
+              type: boolean
+              label: 'Show level labels'
 
 custom_field.view_arguments.*:
   type: string