Add a workflow cancellation management option

CURRENT BEHAVIOR
A workflow cancelled event is sent when a workflow has been abandoned by the workflow initiator or an administrator.

Abandoning a workflow leaves the document(s) the workflow is associated to in its current state, nothing is rolled back.

Currently, nothing is documented regarding this event and it is not clear to users that they could do something to possibly roll back changes made when a workflow is cancelled.

PROPOSAL FOR IMPROVEMENT

• In the definition tab, a new option name "cancellation chain / script" is added.
• Field is optional.
• Help: You may fill here an automation chain or script id that should be called in case the worklow is cancelled. This allows you to roll back changes made during the workflow and prevent the attached document(s) to remain in a non-desired state.

When the field is not blank:

• Studio should generate an event handler that listens to the workflow cancelled event on a regular document, and triggers a chain doing:
  Login as
  Launch chain or script mentioned in the field
  Logout
• The reason behind the login as is to be consistent with the workflow engine: any chain launched in the context of a workflow is executed with admin rights.
• Technically, this event handler is generated upon building the package, it doesn't appear in the UI. Its id should not conflict with an existing event handler.