Suggestion: Add Q_DECLARE_FRIENDS_FOR_FLAGS()

Q_DECLARE_FLAGS() is useful for declaring type-safe flags. However, Q_DECLARE_OPERATORS_FOR_FLAGS() fails if the flag type is a private enum in a class. It would therefore be useful to have Q_DECLARE_FRIENDS_FOR_FLAGS() which would contain all of the necessary friend declarations so that Q_DECLARE_OPERATORS_FOR_FLAGS() could be used for private enums.