Add GTestDBus object

This is a helper to write unit tests using a private dbus-daemon.

session_bus_up/down() are now just wrappers around a GTestDBus singleton.

https://bugzilla.gnome.org/show_bug.cgi?id=672985

gio/giotypes.h

@@ -465,6 +465,8 @@ typedef GType (*GDBusProxyTypeFunc) (GDBusObjectManagerClient   *manager,
                                      const gchar                *interface_name,
                                      gpointer                    user_data);
 
+typedef struct _GTestDBus GTestDBus;
+
 G_END_DECLS
 
 #endif /* __GIO_TYPES_H__ */