Add support for multi-instance call activity to the designer

Currently, the multi-instance call activities which belongs to the BPMN standard elements is missing in RHPAM. We have processes which contains multi-instance call activities, so when we try to import a bpmn diagram into RHPAM, the Designer is not able to show the diagram because of the errors during the importation.

We would like to add this BPMN element in RHPAM in order to be able to migrate our processes from our current BPM to RHPAM, but all the processes which contains a multi-instance call activities fails during the importation.

It seems the engine does support MI CallActivity already:
https://issues.jboss.org/browse/JBPM-4461