Allow to call set_state() on existing routes

Allow to call set_state() on existing routes

Delaylines are not saved in the XML and internal-return is an
invisible processor not explicitly re-added when the state is restored.

They are [re]inserted during Route::setup_invisible_processors().
So this method need to be called after restoring processor state
(indirectly via configure_processors_unlocked as needed).

PS. During route creation this call happens explicitly and on session load
hookup_io() -> Route::output_change_handler() implicitly sets this up.