The choices are now World, View and 3D Cursor.
This breaks Python API compatibility, add-ons that add objects with this
parameter will need to be updated.
Differential Revision: https://developer.blender.org/D4706
- Logical use of fields since they define type information.
- Avoids using ordered-dict metaclass.
Properties using regular assignments will print a warning and load,
however the order is undefined.
