prevent manual start if timer start event is used

According to Maciej's post here https://groups.google.com/d/msg/jbpm-usage/_Ci4WKkhdME/f4rkxIMfBwAJ it isn't supported to manually start a process instance if its start event node is of type "Timer". Besides the fact that this is not documented, the process instance that unsupportedly gets started manually behaves erroneously. Therefore it would be nice to prevent process instances from being started manually whenever the start node is a timer event.