Draft: power: don't show notifications for devices assigned to other seats

Devices that can be assigned to a seat has tag - seat and such devices are always assigned to one seat. If device is not assigned to other seat it is implicitly assigned to seat0.

https://www.freedesktop.org/software/systemd/man/sd-login.html
https://www.freedesktop.org/wiki/Software/systemd/multiseat/

Similar change for gnome-control-center - gnome-control-center!1311