Retrieves a `GtkLayoutChild` instance for the `GtkLayoutManager`, creating one if necessary.
The child widget must be a child of the widget using this.
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`.
| this |
a `GtkLayoutManager` |
| child |
a `GtkWidget` |
|
a `GtkLayoutChild` |