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 set_template
One reason is that generally derived widgets will assume that parent class composite widgets have been created in their instance initializers.
Another reason is that when calling @new on a widget with composite templates, it’s important to build the composite widgets before the construct properties are set. Properties passed to @new should take precedence over properties set in the private template XML.