LibGUI: Split OpacitySlider into vertical and horizontal helper classes

Author: https://github.com/frhun
Commit: 59028515c0
Pull-request: https://github.com/SerenityOS/serenity/pull/16369
Reviewed-by: https://github.com/MacDue ✅

Base/usr/share/man/man5/GML-Widget-OpacitySlider.md

@@ -8,12 +8,13 @@ Defines a GUI opacity slider widget.
 
 ## Synopsis
 
-`@GUI::OpacitySlider`
+`@GUI::HorizontalOpacitySlider`
+`@GUI::VerticalOpacitySlider`
 
 ## Examples
 
 ```gml
-@GUI::OpacitySlider {
+@GUI::HorizontalOpacitySlider {
     name: "opacity_slider"
     tooltip: "Opacity Slider"
 }