Codechange: Replace drop down list's default bools with DropDownOptions. (#14837)

Improves maintainability and avoids positional ambiguity.

src/station_gui.cpp

@@ -664,7 +664,7 @@ public:
 
 			case WID_STL_CARGODROPDOWN:
 				this->filter_expanded = false;
-				ShowDropDownList(this, this->BuildCargoDropDownList(this->filter_expanded), -1, widget, 0, false, true);
+				ShowDropDownList(this, this->BuildCargoDropDownList(this->filter_expanded), -1, widget, 0, DropDownOption::Persist);
 				break;
 		}
 	}