Add Extend setting for wallpaper

[nicola02nb]

Describe the enhancement or feature you'd like
When selecting a wallpaper there should ba also a setting to extend a wallpaper in multiple screens

Describe alternatives you've considered

Additional context

Those are default settings for wallpapers in windwos

[Spiritreader]

This is (sadly) by design. There is an issue with extend mode if set via the API path we're using. If you want to span wallpapers across multiple screens, we recommend you split it into two wallpapers and set it individually for each monitor with the wallpaper (multi monitor) mode

[nicola02nb]

I found a workaround by creating a theme with wallpaper set with extend mode.
Then i setted up the app to switch theme