LT-15797: Split "Referenced Complex Forms" Nodes

- When migrating old Root-based configurations, split
"Referenced Complex Forms" into "Other Referenced Complex Forms"
and "Subentries", restructuring children accordingly.
- Mark custom fields recursively.
- Move DictionaryConfigurationMigrator.AddDictionaryTypeItem
closer to the methods it calls.
- Add Assert message support for certain XPath asserts
- Fix spelling error in "Migrationg"

Change-Id: Id4c4a7f1195c9d39c3febc2c0924268717c926cd