wallpapers/image: avoid using equal in model index comparison
Reason for the change
Equal can confuse the sort filter model when 3 items have the same date. For reversed order Qt swaps the left index and the right index internally, so let's follow the best practice.
Test plan
- Run
kcmshell6 kcm_wallpaper - Choose the slideshow wallpaper plugin and add a bunch of wallpapers
- Choose "Date modified (newest first)"
- Scroll down the wallpaper list to see if any preview is flashing