Feature: Saving background images

[Onetchou]

The user should be able to save the background images into the sm2d pattern file.
The images themselves are not "saved", but their path and all their attributes are, so that the user can work with images, save the pattern, close it, and all the images will appear again the same way they were before saving when the pattern will be opened again (if the images are found at the same place they were when they were imported).