IO/PropStateStore: simplified ctor; tidy.
IO/PropStateStore: added assign_all().
io/prop_state_store: added assign_all() test; tidy.
Object::Unit: set prop state to modified when changing identity.
Object::Unit: added set_scratch_space().
Object: doc tidy.
Object/Ops, IO/Ops: added note about reparenting after Object::unset_parent(); doc tidy.
Object::Unit: set prop state to modified when changing identity or if prop is uninitialized.
Cmd::Node::Create: simplified ResultData.
Cmd::Node::Create: assign modified state to all props after creation.