Userland: Add horizontal mouse scroll support

Author: https://github.com/can3p
Commit: 1662213737
Pull-request: https://github.com/SerenityOS/serenity/pull/11246
Reviewed-by: https://github.com/kleinesfilmroellchen ✅

Userland/Libraries/LibGUI/Scrollbar.cpp

@@ -299,7 +299,7 @@ void Scrollbar::mousewheel_event(MouseEvent& event)
 {
     if (!is_scrollable())
         return;
-    increase_slider_by_steps(event.wheel_delta());
+    increase_slider_by_steps(event.wheel_delta_y());
     Widget::mousewheel_event(event);
 }