wallpaper plug should have basic browsing options

"wallpaper folder" and "custom folder" are fine now, but "picture folders" could be more detailed. I think the user should be able to dig in successive folders within the picture folder and go back the "picture folder root". I believe most people use nested folders to sort their pictures and this renders the selection "picture folder" almost useless compared to "custom folder", even though they would essentially use the same folder.

Only in "picture folders": the folders should be shown, a folder ".." should also be here to go back to the "picture folder root".