Wallpaper config: fix property imageModel that was missing in delegate
Regression was introduced within ScreenLocker highlight settings MR. Used for both desktop and lock screen wallpaper configuration.
Test plan:
- In Desktop Settings Folder / wallpaper : Open containing folder / remove actions should work for each thumbnail
- KCM Screen Locking / Appearance : Open / remove actions should work for each thumbnail, and the current item should be highlighted if different from default value.