Add QImageCleanupFunction getter/setter to QImage

Please add getters and setters to the QImage class to get and set the QImageCleanupFunction and it's void* cleanupInfo.

Right now it is only possible to set a cleanup callback when creating a QImage from raw pixel data. In some cases it can be desirable for a QImage-emitting class, to be notified when the last reference to an emitted QImage 'dies'.

For example, a producer that emits many QImages at a fixed interval and has multiple consumers connecting to it. In this case the producer might want to know when all it's consumers have processed the emitted QImage.