Added a11y initialization code

This basically:
  * Checks a11y configuration properties
  * Checks if clutter has a11y enabled
  * Loads atk-bridge

It also ensure proper NO_GAIL and NO_AT_BRIDGE values on gnome-shell
startup script

https://bugzilla.gnome.org/show_bug.cgi?id=612599

src/shell-a11y.h

@@ -0,0 +1,32 @@
+/* -*- mode: C; c-file-style: "gnu"; indent-tabs-mode: nil; -*-
+ *
+ * Copyright (C) 2010 Igalia, S.L.
+ *
+ * Author: Alejandro Piñeiro Iglesias <apinheiro@igalia.com>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
+ *
+ */
+
+#ifndef __SHELL_A11Y_H_
+#define __SHELL_A11Y_H_
+
+G_BEGIN_DECLS
+
+void shell_a11y_init (void);
+
+G_END_DECLS
+
+#endif /* __SHELL_A11Y_H_ */