Fixed the problems with task history window.
 1. Window automatically updates when an iterator completes.
 2. Iterators that were cancelled before being executed (because
    the first tunable dialog was cancelled) will no longer show
    in the task history window.
 3. TaskHistory class uses the FinishStatus.Type enum instead of
    using its own static byte definitions.
 4. Removed unncessary counting of message types in TaskHistory.History.
 5. In TaskHistory.History.hasMessages(): Replaced
    ConcurrentLinkedQueue.size() > 0 with equivalent !isEmpty().
 6. Added Clean button to clear the task history.
 7. In TaskStatusBar.resetStatusBar(), removed unnecessary invokeLater,
    as this method is always called on the EDT by clearingTimer.

Other task manager issues fixed.
 1. JDialogTaskManager.RunnableTask.innerRun() delegates handling
    of TaskObserver.allFinished() to the run() method's
    finally clause. (Before handling allFinished() was done in run()
    and innerRun().)
 2. run() method updates the task status bar in the finally clause.
 3. Handling the task status bar icon is now done in TaskStatusBar.java.
 4. Close button in the task dialog padding was fixed.

Ref #2463