add showInToolbox parameter into registerElement function

[andrewtelnov]

If there is a need to add a new question type for internal needs that you don't need to see in the Creator Toolbox, then you have to define all items for Toolbox to make sure this new question type is not in the Toolbox.
To avoid it a third parameter, showInToolbox, into the registerElement function is added.
public registerElement(elementType: string, elementCreator: (name: string) => IElement, showInToolbox: boolean = true): void;