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

js/ui/main.js

@@ -88,7 +88,6 @@ function start() {
 
     Gio.DesktopAppInfo.set_desktop_env('GNOME');
 
-    global.grab_dbus_service();
     shellDBusService = new ShellDBus.GnomeShell();
     // Force a connection now; dbus.js will do this internally
     // if we use its name acquisition stuff but we aren't right