Fix crash when approving workflow task

TaskProvider: add the #endTask method. TaskService: change the return type from void to String for the 3 methods #acceptTask, #rejectTask, #endTask. They now return the name of the Seam event to raise efter execution. The unused #notifyEventListeners method was removed. TaskServiceImpl: add the DEFAULT_TASK_PROVIDER constant for the default task provider name. JBPMTaskWrapper: add the JBPM_TASK_PROVIDER constant for the JBPM task provider name. Task: add the TASK_PROVIDER_KEY constant for the task provider variable key. JBPMDocTaskProvider: add the WORKFLOW_REJECT_TRANSITION constant for the workflow "reject" transition. Add 3 classes: AbandonProcessUnrestricted, EndProcessUnrestricted, TaskEventNotificationHelper to externalize and factorize their logic.