LibGUI: Split HorizontalSlider to separate header

This patch splits HorizontalSlider to separate header so it can be used
as a component in .gml files with new GML compiler.

Userland/Libraries/LibGUI/HorizontalSlider.h

@@ -0,0 +1,27 @@
+/*
+ * Copyright (c) 2018-2020, Andreas Kling <kling@serenityos.org>
+ * Copyright (c) 2022, the SerenityOS developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibGUI/Slider.h>
+
+namespace GUI {
+
+class HorizontalSlider final : public Slider {
+    C_OBJECT(HorizontalSlider);
+
+public:
+    virtual ~HorizontalSlider() override = default;
+
+private:
+    HorizontalSlider()
+        : Slider(Orientation::Horizontal)
+    {
+    }
+};
+
+}