Revise XTextbox behavior to address #41
Revise XTextbox behavior to address #41

- Textbox now starts adding text from the top, giving a more intuitive
  operation. Note that the content does not autoscroll once text is
  added below the visible region. Using the scrollbar, the user can
  monitor the latest added rows.

ImpulseAdventure/GUIslice,ImpulseAdventure/GUIslice