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/OpacitySlider.cpp

@@ -139,7 +139,7 @@ void OpacitySlider::mouseup_event(MouseEvent& event)
 
 void OpacitySlider::mousewheel_event(MouseEvent& event)
 {
-    decrease_slider_by(event.wheel_delta());
+    decrease_slider_by(event.wheel_delta_y());
 }
 
 }