Use NuxeoPrincipal instead of Principal in most APIs

The general upgrade rule is the Nuxeo code should not deal with a java.security.Principal anymore (except for internal authentication-related classes). API changes NuxeoPrincipal is now used instead of Principal in the methods and constructors for these classes: CoreSession DocumentEventContext EventContext EventContextImpl InlineEventContext OperationContext QueryFilter SQLQuery.Transformer SecurityPolicy SecurityPolicy.QueryTransformer SecurityService TrashService UnboundEventContext UserWorkspaceService WebContext Removed APIs These APIs were previously deprecated or internal and have been removed. CoreInstance.getInstance() CoreInstance.getNumberOfSessions() CoreInstance.getRegistrationInfos() CoreInstance.openCoreSession(repositoryName, Map context) CoreInstance.openCoreSession(repositoryName, Principal principal) DetachedNuxeoPrincipal SimplePrincipal (use UserPrincipal instead)