Codechange: add and use SetSprite over directly accessing widget_data

src/widget.cpp

@@ -1134,6 +1134,15 @@ void NWidgetCore::SetDataTip(uint32_t widget_data, StringID tool_tip)
 	this->tool_tip = tool_tip;
 }
 
+/**
+ * Set sprite of the nested widget.
+ * @param sprite The new sprite.
+ */
+void NWidgetCore::SetSprite(SpriteID sprite)
+{
+	this->widget_data = sprite;
+}
+
 /**
  * Set the text style of the nested widget.
  * @param colour TextColour to use.