backends/x11: Emit CLUTTER_DEVICE_ADDED events for initial devices

This is similar to commit b9e5a2d6, but for the X11 backend.

Fixes: https://gitlab.gnome.org/GNOME/mutter/-/issues/1466
Fixes: https://gitlab.gnome.org/GNOME/mutter/-/issues/1495

https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1553

(cherry-picked from commit b6211bb6)