public void init_template ()
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
It is important to call this function in the instance initializer of a Widget subclass
and not in constructed or
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.