Universal (across hierarchy) workflow context

Mistral now support "branch" and "global" scope when publishing variables to workflow context. Branch variables are visible only within one branch. Global variables are accessible from any workflow branch within one workflow execution but not accessible from other workflows (and sub workflows). We need to add a new scope where we can publish variables accessible within a hierarchy of workflow executions: a workflow execution and all direct and indirect sub workflows (sub workflows of all levels).

Naming options for this new scope:
* Universal (may be confused with 'global')
* Hierarchy (meaning that it's accessible from a hierarchy of workflow executions)
* Conversation (similar to Spring Webflow)