Export background images and Desklock-screens#289

It would be nice if the used images (background or desklock) could be saved separated from a show (so global/Lampy based and not show-based)

Even better would be an export-mode, so that images from a loaded show can be exported on a stick to use them in a new show.
(or transfered from the show onto a separated map / user_image_map)

They need to be stored inside the show file in order to be exported with the show file; however I can see that export / transfert functionality would be nice to have.

2.2.0 will add exporting of images to USB. We’ll at a later point also add importing from other showfiles.