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

src/shell-global.h

@@ -59,8 +59,6 @@ gboolean shell_global_add_extension_importer (ShellGlobal *global,
                                               const char  *directory,
                                               GError     **error);
 
-void shell_global_grab_dbus_service (ShellGlobal *global);
-
 typedef enum {
   SHELL_STAGE_INPUT_MODE_NONREACTIVE,
   SHELL_STAGE_INPUT_MODE_NORMAL,