Unable to add new directory to slideshow wallpaper setting

[raguse]

@mathochist FYI: You might have noticed in the unstable announcement thread, that qt5ct and qt6ct is about to become obsolete with the introduction of Gnome 42.

@Yochanan

For Qt application theming, we have switched back to QGnomePlatform for GNOME 42. qt5ct and qt6ct (Qt Configuration Utilities) may be marked as orphans. If you want use QGnomePlatform with the Adwaita Qt style, you’ll need remove or comment the QT_QPA_PLATFORMTHEME variable in /etc/environment. You’ll want both qgnomeplatform and qgnomeplatform-qt6 for Qt 5 and Qt 6 applications.