Added options to recurse into source sub-directories, and to use original filenames as base for extracted image files

[virtualnobi]

Francois,

first a disclaimer: This is my first pull request - hope I'll do it as intended. If not, let me know how you want it done.

I liked your Gimp script very much, and added two batch options:

• Recurse through all sub-directories of the source directory
• Use filename of original files as a base for the extracted images (restarting the counter for each)

For that, I had to add a few auxiliary functions, which are placed before the batch version.
I also changed the name from "DivideScannedImages" to "isolate-images-on-background", partly to be able to test both versions in parallel, partly because I thought it better :-)
Finally, there are a few formatting changes, esp. indentation on the script registrations.

If you like to include these changes, I'm happy to provide them. Let me know if you have any questions.

Thanks for all the work you did before - helps me a lot!

Danke & Grüße von
nobi