Initial import of a data model for an LDAP directory to be used with the Interview framework.

There is still much to do, but it's a start. So far the model provides data showing the hierarchical structure of your LDAP directory and the objectClasses of the objects in the directory (tooltips at present).

The model also uses lazy fetching of the data to prevent long startup times.

The eventual plan is for the modle to hold all data of the directory and to filter it by means of proxy models. This will allow us to use the same model to for e.g. Show an overview of the directory structure in one view, and the details of the selected LDAP object in another view. At present the model only searches for the DN and objectClass attributes - this will be extended shortly.

Also added the class LdapDN for easy manipulation of distinguished names.