stack: Only map children when necessary

stack: Only map children when necessary

gtk_widget_set_parent() will map the widget if the parent is mapped
and the widget is both visible and child-visible. As we currently
only set the child visibility after adding the child, we immediately
map all children that are added to a mapped stack, even when they
are not actually shown. Avoid this by setting the child visibility
before adding the child, so widgets are only mapped when shown.

https://bugzilla.gnome.org/show_bug.cgi?id=766737