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>
 * <li>The user makes some changes, saves the file, then undoes the changes.
 * The file <em>is</em> considered dirty: the in-memory state represents an
 * "earlier" version of the file than what is stored on disk.</li>
 * </ul>
 * <p>
 * The activity stack is the primary mechanism for tracking dirty state. There
 * are three values we track:
 * <p>
 * <ul>
 * <li>A transaction counter. Each new transaction (including those that
 * consist of a single command) is assigned a monotonically increasing serial
 * number.
 * <li>
 * <li>The current transaction number. This is the one at the top of the undo
 * stack. If the undo stack size is empty, then it is computed from the
 * transaction counter.
 * <li>
 * <li>The save state of the design. This is the current transaction number in
 * effect at the time the design was last saved. (By definition, the save state
 * and transaction counter are both 0 when a design is created or loaded.)</li>
 * </ul>
 * <p>
 * With the above, it is very easy to detect a dirty design: a design is dirty
 * if and only if its save state differs from the current transaction number.
 * 
 * <h3>Notification</h3>
 * 
 * Any class interested in the activity stack can implement the interface
 * <code>ActivityStackListener</code>. And {@link #addListener}and