Add support for subworkflow node

The sub workflow node is a new kind of node that has a property for referencing a workflow model ID and a way of initializing some global variables of the sub workflow. It should be possible to give static real values, but also to use the current context for transferring values , like referencing ${WorkflowVariables['ma_variable']}. When the workflow engine runs such a node, it starts the other workflow, with the same user session than the one used to enter the node, bound to the same documents list. Then the node is paused and will be resumed as soon as the launched workflowis ended. If the parent workflow is interrupted, the sub workflow has to be interrupted too.
On the sub workflow, the parent workflow instance Id is set as a variable called "parentWorkflow"