A mred:single-panel% is a panel which can contain an arbitrary number of children but only displays one of them at a time. The panel's minimum size is the smallest possible size which still allows each of its children to be fully displayed. These panels are stretchable in both directions by default.
Calls wx:panel%'s super-init.
If parent is an instance of mred:frame% or mred:dialog-box%, calls parent's insert-panel method on itself. Otherwise, it calls parent's add-child method on itself.