Fetch an object build from the template XML for
widget_type in this this instance.
This will only report children which were previously declared with [method@Gtk.WidgetClass.bind_template_child_full] or one of its variants.
This function is only meant to be called for code which is private to the
widget_type which declared the child and is meant for
language bindings which cannot easily make use of the GObject structure offsets.
The `GType` to get a template child for
The “id” of the child defined in the template XML
The object built in the template XML with the id