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 ✅

Userland/Libraries/LibGUI/Forward.h

@@ -33,6 +33,7 @@ class Frame;
 class GroupBox;
 class HeaderView;
 class HorizontalBoxLayout;
+class HorizontalOpacitySlider;
 class HorizontalSlider;
 class Icon;
 class IconView;
@@ -88,6 +89,7 @@ class TreeView;
 class ValueSlider;
 class Variant;
 class VerticalBoxLayout;
+class VerticalOpacitySlider;
 class VerticalSlider;
 class WMEvent;
 class Widget;