Replace dialog doesn't generate thumbnails

The replace dialog doesn't generate thumbnails if one wasn't created in Thunar via scrolling.

It might be good if the replace dialog always generates a new thumbnail, even if a thumbnail exists. It is possible that the existing thumbnail is old, errored, or incorrect in some way. This is where you make a critical decision for a destructive action and comparing files is a very important part of that.

I know the thumbnail system is being worked on in !184 (closed) and this may be part of it, but just wanted to add it in case.