Unexpected Preview mode behaviour on exit

[sidequestboy]

I would expect if a background is previewed and not applied, that it get unset on quit.

[sidequestboy]

There is a problem with "unsetting" the current preview when no saved config exists after pull #33. #35 would allow for setting a solid colour as a fallback in this case.

I think ideally though, we should reset the background to whatever it was before opening Nitrogen... maybe @daf would know how to do that?

[daf]

As of 53135f0, the initial background prior to Nitrogen's launch is preserved and restored. The previous background does not need to be set with Nitrogen in the first place - it will just read whatever's currently there, set by any application.