Minor changes

- Set final blur ratio to 3.
- Add two more user stories.
US23: Add a redo functionality and an Edit menu.

- Add redo action so that undone editions can be redone.
- Add Edit menu to contain both Undo and Redo actions.
US24: Save an index image.

- Save an index image as part of writing the output.
- Improve the creation of the JPEG preview, without a temporary file.
- Refactorings.
US24: Refactorings

- Move the loading of images to ImageStack.
Refactor: Move command line parsing to class Launcher.