Bug fix to handle stepping into a class file.

* When stepping into a constructor during object creation, the class
  file can get loaded. Since there is no source code, we cannot jump to
  a file. Neither can we show any variables. But we should update the
  stacktrace.

* Update the stacktraces whenever a thread is resumed; not just when it
  was a user request. This is because when a thread is sleeping
  (Thread.sleep), when we suspend it and then step over, it gets
  resumed. Here the resume is not caused by user request, but because of
  a step over.