Userland: Make IPC results with one return value available directly

This changes client methods so that they return the IPC response's
return value directly - instead of the response struct - for IPC
methods which only have a single return value.

Userland/Libraries/LibGUI/Desktop.cpp

@@ -59,7 +59,7 @@ bool Desktop::set_wallpaper(const StringView& path, bool save_config)
 
 String Desktop::wallpaper() const
 {
-    return WindowServerConnection::the().get_wallpaper().path();
+    return WindowServerConnection::the().get_wallpaper();
 }
 
 }