desktoppackage: fix unable to apply wallpaper settings after switching layout

`wallpaperGraphicsObject`, `wallpaper` or `wallpaperInterface`
is not set before calling `ContainmentInterface::loadWallpaper()`, so wait
until `wallpaperInterfaceChanged` signal is emitted. At that time
`applyWallpaper()` will call `syncWallpaperObjects()` to update
`wallpaperConfiguration`.

BUG: 407619
FIXED-IN: 5.24.6


(cherry picked from commit 379d162a)