A subwindow<%> is a containee window.
Removes the window from its current parent and makes it a child of new-parent. The current and new parents must have the same eventspace, and new-parent cannot be a descendant of a-subwindow.
If a-subwindow is deleted within its current parent, it remains deleted in new-parent. Similarly, if a-subwindow is shown in its current parent, it is shown in new-parent.