Claim DBus names (Shell, Notifictions) *very* early

Claim DBus names (Shell, Notifictions) *very* early

See commit f2158218bef0c51 in mutter.  Basically, we need
to grab org.freedesktop.Notifications before anything else
in the session gets started.

Note: I intentionally removed the Util.killall bits.  I believe that
for notification-daemon at least, if we specify
DBUS_NAME_FLAG_REPLACE_EXISTING, we'll take over the name.  Not sure
about notify-osd; if that's still a problem, then what we need to do
is add killing (and possibly respawning) of notify-osd to
"gnome-shell --replace", and not have it embedded randomly in a JS file.

https://bugzilla.gnome.org/show_bug.cgi?id=642666