Sync Wallpapers to Greeter

[danirabbit]

Currently we have code here: https://github.com/elementary/switchboard-plug-pantheon-shell/blob/master/set-wallpaper-contract/set-wallpaper.vala

and here: https://github.com/elementary/switchboard-plug-pantheon-shell/blob/master/src/Views/Wallpaper.vala#L206

for making sure wallpapers get synced to the greeter. We should move that to the settings daemon so that things that touch the wallpaper only have to interact with gsettings

I guess we should consider if the "copy to library" code makes sense here as well to make sure wallpapers aren't accidentally deleted