Add @enum and @ref to top-level version

* base action for types.Mistral.Action is either a predefined action
  (currently hardcoded) or already existing types.Mistral.Action
  entity with non-empty name (except the current one)
* Action, Task, Workflow and Workbook 'version' field are all just
  references to a single types.Mistral.Version instance

Change-Id: I3fe354c64cf4361e7dff8686e848cec3448d315f
Merge "Add @enum and @ref to top-level version"