Allow processRequest, getObject, et al, to return IRequestProcessor instances

[nevali]

Rather than solely relying on forwarding, getObject() (and by extension, processRequest()) should be able to return an IRequestProcessor instance to cause the active Request object to be processed with that instance instead. This would not be a valid return for anything intended to be a top-level App instance, as that would obviously alter the basic request-processing flow in platform.php and index.php