qtprotobufgen: oneof fields provide mutable accessors

message ChatMessage {
    string username = 1;
    int64 timestamp = 2;
    oneof content {
        TextMessage text = 3;
        FileMessage file = 4;
        UserStatus user_status = 5;
    }
} 

    QPB_QTCHAT_CLIENT_PROTO_EXPORT bool hasText() const;
    QPB_QTCHAT_CLIENT_PROTO_EXPORT TextMessage &text() const;    QPB_QTCHAT_CLIENT_PROTO_EXPORT bool hasFile() const;
    QPB_QTCHAT_CLIENT_PROTO_EXPORT FileMessage &file() const;