wallpaper/slideshow: shouldn't display "current" item in image list

In the config dialog of slideshow wallpaper, the first image is always in a "current selected" state (thick, deep blue border). And it can't change with neither mouse click or keyboard.

Slideshow has no such thing as "current", so this effect should be removed.

BUG: 457327

![Screenshot_20220731_112227](/uploads/73be3edcb45ac3a1ef5c92b26f6cf46e/Screenshot_20220731_112227.jpg)


(cherry picked from commit 01defdb5)