Small fix in GUI

After I noticed that moving image examples and removing some of them caused qim3d to crash, I decided to make it more dynamic and less hard coded. The new version should be stable if someone decides to change some of the examples. The folder should remain with the same name.

While at it, I made a small change in local thickness description we talked about. Saying that it accepts 2D images is misleading. Thus I deleted this small part.