Right now, if the user sets a wallpaper and it disappears on disk, the next time Plasma
is restarted, the wallpaper will be reset to the default, which is probably not what
the user expected.
This patch caches the wallpaper locally every time the user specifically changes it.
Thereafter, if the path to the original wallpaper becomes invalid, Plasma will fall
back to reading the wallpaper from the cache.