output: move a few calls from new_output_notify()

output: move a few calls from new_output_notify()

... to more specific/logical locations, to make it possible in future to
*not* immediately auto-configure new outputs when connected.

In particular:

- Move regions_reconfigure_output() and session_lock_output_create()
  into add_output_to_layout(), which would be called also if the output
  is enabled later.

- Move seat_output_layout_changed() to do_output_layout_change() so it
  is called for any layout change, not just adding new outputs.