Add scrollbaroptions FormSpec element (#8530)

src/gui/guiScrollBar.h

@@ -23,6 +23,12 @@ public:
 	GUIScrollBar(IGUIEnvironment *environment, IGUIElement *parent, s32 id,
 			core::rect<s32> rectangle, bool horizontal, bool auto_scale);
 
+	enum ArrowVisibility {
+		HIDE,
+		SHOW,
+		DEFAULT
+	};
+
 	virtual void draw();
 	virtual void updateAbsolutePosition();
 	virtual bool OnEvent(const SEvent &event);
@@ -39,6 +45,7 @@ public:
 	void setLargeStep(const s32 &step);
 	void setPos(const s32 &pos);
 	void setPageSize(const s32 &size);
+	void setArrowsVisible(ArrowVisibility visible);
 
 private:
 	void refreshControls();
@@ -47,6 +54,7 @@ private:
 
 	IGUIButton *up_button;
 	IGUIButton *down_button;
+	ArrowVisibility arrow_visibility = DEFAULT;
 	bool is_dragging;
 	bool is_horizontal;
 	bool is_auto_scaling;