Bug 148755 Add tthe status to the ActivityStackEvent.
If the the event rollback, the ActivityStackEvent status should be ROLL_BACK.
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/activity/ActivityStack.java
* still be deleted, and no new element can appear in the model with the same
 * name as the deleted element. These constraints allow the record to reverse
 * its effect without fear of invalidating the design. The result is that the
 * model transitions back to the initial state.
 * <p>
 * This careful transition of states works only if records are executed, undone,
 * and redone in exactly the right order. Records must be undone in the order
 * that they were done. Suppose that they are not. In this case, when record C
 * is undone, it will find that the model is not in the expected final state for
 * record C, and thus there is no guarantee that doing the undo will leave the
 * model in either the initial state or even a valid state.
 * <p>
 * It is for this reason that executing a record must flush the redo stack. The
 * effect of the new record is to move the model to a state different than the
 * final state of the first record on the redo stack. Hence, that record can no
 * longer be redone and must be flushed.
 * <p>
 * Similarly, the record system works only if all changes to the model are done
 * through records. Suppose that some change is made without a record. The
 * result is that the model has changed from state Sn to state Sn+1. However,
 * the record at the top of the undo state still expects the model to be in its
 * final state: state Sn.
 * 
 * <h3>Notifications</h3>
 * 
 * The stack is responsible for sending out notification events as records
 * execute (do, undo or redo). Each record sends out a notification event at the
 * request of the stack. ActivityRecord in the stacks have a state that must
 * match the stack on which they appear. The state is used to compute
 * notification events for the record.