Creates and initializes child widgets defined in templates.
This function must be called in the instance initializer for any class which assigned itself a template using [ method@Gtk.WidgetClass.set_template].
It is important to call this function in the instance initializer of a `GtkWidget` subclass and not in `GObject.constructed()` or `GObject.constructor()` for two reasons:
- derived widgets will assume that the composite widgets defined by its parent classes have been created in their relative instance initializers
- when calling `g_object_new()` on a widget with composite templates, it’s important to build the composite widgets before the construct properties are set. Properties passed to `g_object_new()` should take precedence over properties set in the private template XML
A good rule of thumb is to call this function as the first thing in an instance initialization function.