Code review comment for lp:~mterry/ubuntu-system-settings/default-wallpaper

Oooh, had a thought regarding the system-wallpaper-copy issue. What about this:

- When switching wallpapers, if the old wallpaper is still on the system and we also keep a copy, delete the copy. This keeps us from keeping copies of all the wallpapers, if the user tries them all out.
- If we have a copy of a system wallpaper, but it is no longer on the system, treat that wallpaper as a custom wallpaper. That way, the user can delete it.

Thus we have at most one undeleteable wallpaper copy per user. The disadvantage is that it is harder to get back a wallpaper you used to like after we upgrade distros. But that's always true in that case.

Unless I hear otherwise, I'll work on this approach.