function ButtonText(properties) {
                        if (properties)
                            for (var keys = Object.keys(properties), i = 0; i < keys.length; ++i)
                                if (properties[keys[i]] != null)
                                    this[keys[i]] = properties[keys[i]];
                    }
Constructs a new ButtonText.
@memberof proto.Message.ButtonsMessage.Button
@classdesc Represents a ButtonText.
@implements IButtonText
@constructor
@param {proto.Message.ButtonsMessage.Button.IButtonText=} [properties] Properties to set