Retrieves a GtkLayoutChild instance for the GtkLayoutManager,
creating one if necessary.
The @child widget must be a child of the widget using @manager.
The GtkLayoutChild instance is owned by the GtkLayoutManager,
and is guaranteed to exist as long as @child is a child of the
GtkWidget using the given GtkLayoutManager.
Retrieves a GtkLayoutChild instance for the GtkLayoutManager, creating one if necessary.
The @child widget must be a child of the widget using @manager.
The GtkLayoutChild instance is owned by the GtkLayoutManager, and is guaranteed to exist as long as @child is a child of the GtkWidget using the given GtkLayoutManager.