center in top-level-window<%>

Centers the window on the screen if it has no parent. If it has a parent, the window is centered with respect to its parent's location.

• (send a-top-level-window center direction) -> void
     direction = 'both : symbol in '(horizontal vertical both)

  If direction is 'horizontal, the window is centered horizontally. If direction is 'vertical, the window is centered vertically. If direction is 'both, the window is centered in both directions.