Adjust scenario editor scroll bar size/positioning.

FIxes #772

src/dialogxml/widgets/scrollbar.cpp

@@ -87,6 +87,10 @@ void cScrollbar::setWheelEventRect(rectangle rect) {
 	this->wheel_event_rect = rect;
 }
 
+rectangle cScrollbar::getWheelEventRect() const {
+	return wheel_event_rect;
+}
+
 long cScrollbar::getPosition() const {
 	return pos;
 }

src/dialogxml/widgets/scrollbar.hpp

@@ -125,6 +125,7 @@ public:
 	/// @param newStyle The new style.
 	void setStyle(eScrollStyle newStyle);
 	void setWheelEventRect(rectangle);
+	rectangle getWheelEventRect() const;
 	virtual void draw() override;
 	virtual bool handle_event(const sf::Event&) override;
 	/// @copydoc cControl::getSupportedHandlers