wallpapers/image: add select all, unselect all buttons for slideshow mode

Reason for the change

When working with many wallpapers, it is hard to rapidly unselect the images that should be used for the slideshow, this makes it tedious to view individual or small groups of images in a slideshow.

Test plan

I tested the changes by opening "Wallpaper" in system settings, making sure that the selection options don't show in modes other than "Slideshow"

Screenshots or screen recordings

Before	After

Bugs fixed

FEATURE: 484779
FIXED-IN: 6.5