In progress #6
* Added `WPDKObject::delta()` public static method to replace `wpdk_delta_object()` the inline (deprecated) function
* Added `WPDKResult::isError()` public static method to replace `wpdk_is_error()` the inline (deprecated) function
* Added `WPDKResult::isWarning()` public static method to replace `wpdk_is_warning()` the inline (deprecated) function
* Added `WPDKResult::isStatus()` public static method to replace `wpdk_is_status()` the inline (deprecated) function