Refactoring/dataset frame preview

I relocated from the DialogConvolve class to Dataset the code that used to perform the frame preview. To do so, the part of the code that was applying the threshold has been removed. If we want to keep it, we then would have to add a flag to Dataset::convolvedFrame method which will defin whether or not applying the threshold. In doing that work I had to refactor but also debug quite deeply the whole dialog. I think it is much better shaped now. Tell me if you (dis)like the changes.