Check type in SyncDataType's DataTypeDefintion_AfterSave handler to avoid exception when saving DocumentType

The AfterSave event is defined on CmsNode so the sender isn't guaranteed to be a DataTypeDefinition instance.
Reintroduce working code in DataTypeDefinition_Saving handler

The DataTypeDefinition_AfterSave handler doesn't get hit when a data type is saved in 6.1.2.
Merge pull request #3 from dpen2000/FixExceptionSavingDocumentType

going to test on v4 / v6 versions and push a new release out