Encapsulate IA2 relations constants (PR #13096)
* Relations constants
Compat breaking refactor:
- Removed IA2_RELATION_FLOWS_FROM
- Removed IA2_RELATION_FLOWS_TO
- Removed IA2_RELATION_CONTAINING_DOCUMENT
- Replaced with IAccessibleHandler.RelationType enum
* Breaks compatibility in 2022.1
