Merge appearance and background

[plfiorini]

Not sure how the layout will look like though.

appearance has a pane with different sections, one of them is Lock Screen guess that it was already meant to have lock screen appearance settings.

One way could be to have a Background section instead with two list items: desktop and lock screen that acts just like the background module buttons. Clicking on the item will open a modal dialog (not a separate window, rather the Popup control) where you can select the background.