pw: Make Permission editable

Add a constructor, setters, and derive Clone on Permission.
pw_permission's fields are all public, this reflects that.
Clone (maybe make it Copy too?) is useful for turning the &[Permission] slice gotten from the permissions event on Clients into an owned Vec for example.
The constructor and setters are useful for Client::update_permissions. One might want to add permissions to the previously mentioned Vec or modify an element in that and pass it as the slice.