Codechange: add and use GetToolTip instead of direct access

src/widget.cpp

@@ -1154,6 +1154,15 @@ void NWidgetCore::SetToolTip(StringID tool_tip)
 	this->tool_tip = tool_tip;
 }
 
+/**
+ * Get the tool tip of the nested widget.
+ * @return The tool tip string.
+ */
+StringID NWidgetCore::GetToolTip() const
+{
+	return this->tool_tip;
+}
+
 /**
  * Set the text/image alignment of the nested widget.
  * @param align Alignment to use.