"Compare" utility. Some suggestions (juxtaposition)

[Zheitk]

- Add an overlay button, useful for easily comparing the quality of the pics.
- The possibility to compare individual files (if you have 3 selected, it should open the comparing dialog with those 3 files)
- In the case that one picture were smaller than another, add an auto-zoom option (to match the smaller image to the large one), so you can pan and compare the pics easily.