Updated with two bug fixes

In this release I've fixed two bugs:

1. If you click Write images to disk before you've clicked Slice, the application will now slice the images first and then write the image files to disk whereas the previous version displays an error message dialog box.

2. The previous version has a bug whereby if your source image has gaps, the writing process will write out the correct files but continue scanning the image file in the background - it's not noticeable to the user and completely harmless but the error was there nonetheless.