property: removed modify callback

this partially reverts 0e065df794966790f15a1f8833b1c09299e12be8
relationships: added getRelationshipMapper method
relationships: added IRelationshipMapper::isStoredInEntity() (interface BC break!)
relationships: set as modified only if relationship mapper says so
relationships: fixed HasOne::isChanged() comparison

tested by RelationshipsHasOneIsChangedTest, which covers this
becaus current changes are loading entity, which can't be loaded
due to unattached state.