Track XKB group configuration change

Since GNOME 3.6, switching XKB layouts changes the group
configuration, while libcaribou assumes that the configuration is
fixed during the session.

This patch tries to track group configuration changes and allow
clients to reconstruct keyboard UI through two new signals:
KeyboardModel::group-added and KeyboardModel::group-removed.

https://bugzilla.gnome.org/show_bug.cgi?id=694011