WindowServer: Rename WallpaperMode::{Scaled => Stretch}

This option was renamed from scaled to stretch in DisplaySettings in
699ba84, but since WindowServer receives a plain string and was not
updated, it wouldn't recognize the new renamed value as a valid option.
Turns out sending plain strings via IPC and only mapping them to enum
values on the receiving end is brittle, we should probably update
Desktop::set_wallpaper_mode() to use an enum as well at some point.

Fixes #5006.

Userland/Services/WindowServer/Compositor.h

@@ -43,7 +43,7 @@ enum class WallpaperMode {
     Simple,
     Tile,
     Center,
-    Scaled,
+    Stretch,
     Unchecked
 };