[schema migration] make some stuff to ease file 1.9 migration : we want to kill the Image entity so that existing image are turned into (existing entity type) File entities

[entity] continue cleanup of Entity/AnyEntity namespace * prefix by cw_ or _cw all '2nd zone' attributes / methods, other most commonly used will be done later (some methods used by client are made private anyway when they are for internal purpose anyway) * kill set_eid, use direct affectation instead * kill the auto-transmutation feature (pre_add_hook), hence releasing 3.9 will require releasing file cube as well