additional packed data location for automatic backup of unsaved blender edited images

A problem often encountered is loss of data when images are lost, either because of a crash or when forgetting to save an image manually before closing Blender. Potentially losing hours of work.

A solution would be to have a second packed data location internally which would automatically save the image during autosave, or when closing Blender to avoid loss.

If the image being edited is already manually packed, then Blender would use the copy in the packed backup only if it's newer.

The image editor could have an option to 'revert to manually saved version' to avoid being forced to use the newer auto packed version.