Fix ArrangeWindowsStack* for multi-head displays

* Fix integer wrap around for some situations (resulting in a very wide
  window):

      w = max_width - x_offs - (*closest)->widthOffset()

  If the given window is on the right most display and thus 'x_offs' is
  bigger than 'max_width' (half of the display width), the resulting 'w' wraps
  around and becomes very large.

* Place a single window via ArrangeWindowsStack* as well
* Some minor code styling / reordering along the way