Use CF convention for methods returning unmanaged objects

Static analyzer panics when retained non-managed objects returned from methods that do not follow CF naming convention (e.g. all methods that return retained object must start with "new" keyword in their name).

While I do not want to rename methods, I simply used `CF_RETURNS_RETAINED` attribute to achieve the same effect.

Thanks for the great label subclass, I use it for many years in my apps.