backends/native: Wait to have an stage before emitting CLUTTER_DEVICE_ADDED

During seat initialization, we process early libinput events (adding all known devices) before the seat gets a stage assigned. This causes warnings when trying to handle the corresponding CLUTTER_DEVICE_ADDED events, as they are sent stageless.

As it is definitely too soon to have those events sent meaningfully, filter those events out and instead handle the CLUTTER_DEVICE_ADDED emission for all known devices after the seat receives an stage. This makes the events guaranteed to be emitted early in initialization, but not so soon that they can't be handled yet.