Fixed ImageViewNext size "jumping" when loading

ImageViewNext now shows the Loading image instead of the Empty (which defaults to a color) while it's asynchronously deserializing and loading an image (using ImageViewEx#setSource), so that there is no size flickering issue on non-fixed size ImageViewNexts -- provided, of course, that the loading image has the same size of the final image, but there's nothing we can do to predict the future, I guess.