Description of problem:
User can set 'Is Async' property for variety of tasks (Manual, User ...). But this property is missing for script tasks. In web jbpm-designer is possibility to set 'Is Async' property also for script tasks.

Version-Release number of selected component (if applicable):
jbds-9.1.0.CR1_jbdsis-9.0.0.CR1
BPMN2 Modeler - 1.2.4.Final-v20160330-1625-B110

Steps to Reproduce:
1. Create new process
2. Add 'Script Task' to canvas
3. Open properties of task from step 2.
4. Select 'Script Task' tab

Actual results:
There is missing 'Is Async' property on selected tab

Expected results:
There is present 'Is Async' property on selected tab