Remaining data object classes: DXFile, DXGTable, DXApplet, DXWorkflow

* DXGTable, DXFile, DXApplet, and DXWorkflow are mostly intended for
  obtaining handles to existing data objects. For example, no high-level
  bindings are provided for uploading or downloading file data, or
  adding or getting GTable rows.
* Add JSON (de)serializers for data object classes.
* Support for test configuration variables.