* provide shuffled view.

* In the course of that, I was almost tempted to provide a shuffle() method on DB::Result, manipulating the list.
However that would be not a good idea, because the list passed in to launchViewer() is actually used by the ThumbnailViewer, so shuffling that original list would be bad.

To confine that potential problem, the list returned by the ThumbnailWidget is a ConstResultPtr, thwarting all attempts to do something non-const on the list.

ConstResultPtr should be used more often in cases we return a ResultPtr whose contents must not be modified.