If a gtk.layout_manager.LayoutManager has per-child properties, like their packing type,
or the horizontal and vertical span, or the icon name, then the layout
manager should use a gtk.layout_child.LayoutChild implementation to store those properties.
gtk.layout_child.LayoutChild is the base class for objects that are meant to hold layout properties.
If a gtk.layout_manager.LayoutManager has per-child properties, like their packing type, or the horizontal and vertical span, or the icon name, then the layout manager should use a gtk.layout_child.LayoutChild implementation to store those properties.
A gtk.layout_child.LayoutChild instance is only ever valid while a widget is part of a layout.