Clean up the widget activation mess
We still have the ancient activate_signal mechanism, even though we've introduced a lot of action machinery in GTK4.
There's various ways to activate a widget: activate, activate-default and mnemonic-activate.
We should replace the activate_signal with an action, port all users to it, and get rid of activate_signal. There are also apis like gtk_widget_class_add_binding_signal, gtk_signal_action_new()