Remove link to parent object in GffInstance
Redo Dialog object.

- Moved to property generators.
- Combine DialogStart and DialogPointer.
Update objects

- Update to new GffInstance.
- Rename all save() functions to stage() so it's more clear
  that calling it is not *saving* anything at the time of call.
- Add stage() functions to instance objects and parameters pointing
  back to their parent object.
- Add a few more functions here and there.
Document structure update.

I have a feeling this could be done a much better way...