Adds Item and PropertyLookup interfaces

Allows libraries that use WikibaseDataModel to type against a shared interface to retrieve entities.

It would allows libraries that do manipulation on entities (example: conversion to an RDF based representation) to load dynamically entities without having to depend directly on a storage implementation.

Example of use case: move RdfBuilder out of the Wikibase extension into a standalone library that only depends on WikibaseDataModel and EasyRdf