BUG corrects MoneyField::setReadonly() method in order to return ReadonlyField instances of its child fields

Issue #2766 - This causes our cloned MoneyField to actually contain two ReadonlyFields for currency and amount.
Issue #2766 - Moves logic to return ReadonlyField versions of MoneyFields child fields to performReadonlyTransformation() method and reverts changes to MoneyField::setReadonly() as per Wilr’s (https://github.com/wilr) request
Merge pull request #2767 from HARVS1789UK/2766-moneyfield-set-readonly

BUG: #2766 Corrects MoneyField::performReadonlyTransformation()