When the session goes away, e.g. seatd being stopped at shutdown, wlroots
destroys the backend by itself (wlr_auto_backend_monitor), and
wlr_backend_finish() aborts since dwl's new_output and new_input listeners
are still attached:
Assertion failed: wl_list_empty(&backend->events.new_input.listener_list)
Detach them and quit instead.
