Declares a @callback_symbol to handle @callback_name from
the template XML defined for @widget_type.
This function is not supported after [method@Gtk.WidgetClass.set_template_scope]
has been used on @widget_class. See [method@Gtk.BuilderCScope.add_callback_symbol].
Note that this must be called from a composite widget classes
class initializer after calling [method@Gtk.WidgetClass.set_template].
Declares a @callback_symbol to handle @callback_name from the template XML defined for @widget_type.
This function is not supported after [method@Gtk.WidgetClass.set_template_scope] has been used on @widget_class. See [method@Gtk.BuilderCScope.add_callback_symbol].
Note that this must be called from a composite widget classes class initializer after calling [method@Gtk.WidgetClass.set_template].