feat: partial implementation of wallpaper settings

Will take some time to refactor the rest

app/src/pages/mod.rs

@@ -15,6 +15,7 @@ pub enum Message {
     DateAndTime(time::date::Message),
     Desktop(desktop::Message),
     Panel(desktop::panel::Message),
+    DesktopWallpaper(desktop::wallpaper::Message),
     External { id: String, message: Vec<u8> },
     Page(Entity),
 }