wallpapers/image: save drag and drop result after `addSlidePath`
Fixes binding loop warnings when launching plasmoidviewer.
file:///home/ratijas/kde/usr/share/plasma/wallpapers/org.kde.image/contents/ui/main.qml:62:5: QML ImageBackend: Binding loop detected for property "slidePaths"
file:///home/ratijas/kde/usr/share/plasma/wallpapers/org.kde.image/contents/ui/main.qml:62:5: QML ImageBackend: Binding loop detected for property "slidePaths"