Improved image sorting.

[ChristopherFritz]

Sorts images by folder then file name.

Addresses issue #25.

This was tested with files named as shown below.

Sort before code change:

• Chapter 1/0003.jpg
• Chapter 20/0051.jpg
• Chapter 69/13.jpg
• Chapter 69/3.jpg
• Chapter 9/0079.jpg

Sort after code change:

• Chapter 1/0003.jpg
• Chapter 9/0079.jpg
• Chapter 20/0051.jpg
• Chapter 69/3.jpg
• Chapter 69/13.jpg

Disclaimer: I don't really know Python, so I figured things as out I went to try and address this issue.

[ChristopherFritz]

Afterthought: I should have separate sort_width variables for folders and images, but the way it's written doesn't hurt any.

[kha-white]

I decided to use the natsort library after all, but thank you for your contribution, and sorry for the delayed repsonse.