create AbstractHibernateExtension and make HibernateExtensionX_Y inherit from it
pull up instance variables to AbstractHibernateExtension
build service registry in ConfigurationProxy.buildSessionFactory() for plugin org.jboss.tools.hibernate4_3
remove use of org.hibernate.service.ServiceRegistry in HibernateExtension4_3
build service registry in ConfigurationProxy.buildSessionFactory() for plugin org.jboss.tools.hibernate4_0
remove use of org.hibernate.service.ServiceRegistry in HibernateExtension4_0
remove IConfiguration.buildSessionFactory(Object)