Add: AppendStringInPlace() to append translated string ID into an existing string. (#12969)

This allows avoiding a string copy when building strings.

src/depot_gui.cpp

@@ -896,7 +896,7 @@ struct DepotWindow : Window {
 			SetDParam(1, loaded[cargo_type]);   // {CARGO} #2
 			SetDParam(2, cargo_type);           // {SHORTCARGO} #1
 			SetDParam(3, capacity[cargo_type]); // {SHORTCARGO} #2
-			details += GetString(STR_DEPOT_VEHICLE_TOOLTIP_CARGO);
+			AppendStringInPlace(details, STR_DEPOT_VEHICLE_TOOLTIP_CARGO);
 		}
 
 		/* Show tooltip window */