* Add WORKFLOW.{scripts,transitions...} property which is just a ComputedAttribute
  returning a dict (TODO: Instead of a dict, this should be ContainerTab to have
  objectIds()...).

* Allow a Workflow Script to call another one by overriding portal_workflow.__getattr__.
  With DCWorkflow, `container` was bound to its parent (WORKFLOW.scripts which is a
  mapping), but now `container` is bound to the WORKFLOW itself as Transitions, Scripts
  and Variables are all at the same level. This is not very efficient but this is only
  in DCWorkflow compatibility mode after all.

  The initial implementation was creating a `ScriptContext`, a temporary object with
  all scripts of the current Workflow added. However, this required changing existing
  Workflow Script code and especially this did not work with the following use case:
  1. Script Context is created before calling a Workflow Script.
  2. That script calls a Workflow Script from another Workflow.
     => This will fail as ScriptContext is only created for the Workflow.

This commit is going to be squashed before merging to master.