nd-daemon: Add a separate notification-daemon

Add a small service that exposes the Fdo notification API under the
well-known name, and forwards any requests to the actual implementation
in the shell.

That way any app with permission to talk to org.freedesktop.Notifications
will get exactly that, and nothing more.

GNOME/gnome-shell!547
4 jobs for !547 with notification-daemon in 3 minutes and 40 seconds (queued for 6 seconds)
latest detached
Status Job ID Name Coverage
  Review
passed #302063
check_commit_log

00:00:20

 
  Build
passed #302064
build

00:01:54

 
  Test
passed #302066
test

00:01:22

passed #302068
test-pot

00:01:24