Add invariant that a child is unmapped if parent is unmapped

Requires fixes to GtkContainer and GtkWindow to unmap their
children, rather than just withdrawing or hiding the container
window.

Requires fix to GtkHandleBox to chain up to GtkContainer unmap.

Historically we avoided these unmaps for efficiency reasons,
but these days it's a bigger problem that there's no way
for child widgets to know that one of their ancestors has
become unmapped.

docs/widget_system.txt

@@ -255,7 +255,7 @@ In the following
 
    widget->parent && GTK_WIDGET_MAPPED (widget->parent) && 
      GTK_WIDGET_VISIBLE (widget) && GTK_WIDGET_CHILD_VISIBLE 
-       => GTK_WIDGET_MAPPED (widget)
+       <=> GTK_WIDGET_MAPPED (widget)
 
 Note:, the definition