if GetStringHelper is set in Menu<std::string>, invoke it

previously it always returned raw string and ignored this
helper function, but there might be cases when one would
want to use its own function e.g. to parse the string somehow.

src/menu.cpp

@@ -47,5 +47,14 @@ bool List::Deselect()
 
 template <> std::string Menu<std::string>::GetOption(size_t pos)
 {
-	return itsOptionsPtr->at(pos) ? (*itsOptionsPtr)[pos]->Item : "";
+	if (itsOptionsPtr->at(pos))
+	{
+		if (itsGetStringFunction)
+			return itsGetStringFunction((*itsOptionsPtr)[pos]->Item, itsGetStringFunctionUserData);
+		else
+			return (*itsOptionsPtr)[pos]->Item;
+	}
+	else
+		return "";
 }
+