rework updating the image in the preview viewer

Next merge request in this series. This one aims to decouple the updating of the preview image from KSaneScanThreads member functions, which will not be accessible once logic and interface are split.

KSaneScanThread::saneStartDone() is removed. Instead, the progress is only updated after the initial setup of the scan has been done. This ensures that the scanning has started and the QImage has been set up properly before updating the QImage for the preview viewer.

Furthermore, KSaneScanThread::imageResized() has also been removed. The conditions to set up or update the width and height preview viewer are now determined based on the actual width and height of the QImage.