Shows or hides a window.
The visibility of a top-level window can be changed by the user clicking the window's close box, and such changes do not go through this method; use on-close to monitor visibility changes.
If show? is #f, the window is hidden. Otherwise, the window is shown.
If the window is already shown, it is moved front of other top-level windows. If the window is iconized (frames only), it is deiconized.