Sets the current object for the this.
The current object can be thought of as the `this` object that the builder is working for and will often be used as the default object when an object is optional.
[method@Gtk.Widget.init_template] for example will set the current object to the widget the template is inited for. For functions like [ ctor@Gtk.Builder.new_from_resource], the current object will be null.
the new current object