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
* 
 * <li>While the record executes, is undone, or is redone, the model may
 * temporarily be in an invalid <em>transition</em> state. No application
 * logic should execute during the transition state. This state must be seen
 * <strong>only </strong> by the record itself, since only this record
 * understands this state.</li>
 * <li>The undo of a record must transition the model from the final state back
 * to the initial state.</li>
 * 
 * <li>The redo of a record must again transition the model from the initial
 * state to the final state.</li>
 * 
 * <li>Suppose the application issues three records in succession: C1, C2 and
 * C3. Then the final state of C1 is the initial state of C2. And the final
 * state of C2 is the initial state of C3.</li>
 * 
 * <li>Whenever a new record is executed, the redo stack is flushed.</li>
 * 
 * <li><strong>All </strong> changes to the model are done by records.</li>
 * </ul>
 * <p>
 * To understand why these rules are important, consider how records work.
 * Suppose we have a record that is to delete an element E. Obviously, for the
 * record to work, element E must exist and must be part of the model. This is
 * the initial state, and is a valid model state. When the record executes,
 * element E must be removed from the model and any dangling references must be
 * cleaned up. The result is a final state that is also a valid state.
 * <p>
 * In order for the above record to undo, the exact same final state must obtain
 * at the time we undo the record. That means, for example, that element E must