Add event to Open UI API for document save, auto-save and cancel/discard

Now that Open UI document fields can read values from other document fields, it's become a valid use case to have an Open UI field display something or base its behavior on another field's value. However, it's not possible to automatically update the Open UI field when the field it depends on changes value.

Scope of this issue:
Add subscribable events for document save (to unpublished), document auto-save (to draft) and discard changes that Open UI documents fields can use.