CRM_Utils_Migrate - Handle CustomGroup subtypes

When using "civix generate:custom-data" with CRM_Utils_Migrate_Export and CRM_Utils_Migrate_Import, there are various problems when handling custom-data groups with subtypes (e.g. CustomGroups for RelationshipType or ActivityType) – e.g. "activity_type" OG can't be meaningfully exported/imported in this case (because activity_type_id's would conflict on new installations).

For hremerg and CiviVolunteer, we worked around this by using the installer to create the reltype/acttype and using Smarty to modify the CustomData XML, but this approach currently requires too much manual involvement.

When completed, CRM_Utils_Migrate_Export::buildCustomGroups() will produce portable XML documents that can be used by the CRM_Utils_Migrate_Import to create custom-groups which use subtypes (even if the subtypes have diff id's).