Display important notifications on the top layer

[FriederHannenheim]

I have noticed that when I have a video fullscreen, notifications don't appear. This is sometimes great as I do not want to be disturbed, however sometimes my Laptop just shuts down because I missed the "BATTERY CRITICAL" notification.
It would be great to have notifications with urgency = critical show up above all apps so you can't miss them.

I would like to implement this and can do a PR but I need some feedback first. Maybe it would be better to have a setting for the wlr_layer the notifications should appear on but that would then apply to all of them.