[system settings] Not clear that the background has actually been changed

[Matthew Paul Thomas]

During usability testing in August, when asked to change the background picture, a few participants were not sure that they had succeeded.

In the original design, this was handled by the top-level "Backgrounds" screen showing thumbnails of the lock screen background, and the Dash background. As soon as you changed the background, you ended up back at this screen.

Now that we don't have a Dash background, that top-level screen doesn't exist any more, so we need some other way of showing the change.

<https://wiki.ubuntu.com/Appearance>: "If you choose “Set”, the background should be set, and the zoomed and panned area of the image should animatedly shrink down to become the first (and selected) item in the “Recent Images” section, with the screen scrolling to reveal it if necessary."

[Daniel van Vugt (vanvugt)]

I would (naively as a regular user) expect one of two things to happen:
   (a) the dash background to use the selected wallpaper; or
   (b) [at least] the wallpaper to be visible behind the window spread.

Neither happens so any reasonable user is left confused. Also maybe annoyed that they don't get to see their favourite wallpaper photo while moving through the dash.