[GUI] More notes on the example implementation

[Wolfgang Thaller]

Glynn Clements wrote:

> Wolfgang Thaller wrote:
>
>> *) We might need to add the parent widget as a required parameter to
>> the constructor functions.
>
> Xt requires the widget's parent to be specified at creation time, and
> doesn't allow the parent to be changed thereafter.

So unless we want to defer creation of the native widget somehow, we 
have
to add the parent as a parameter to the constructor.
The HasAddChild class could be renamed to something like "Container",
and the addChild method has to go away.

Cheers,

Wolfgang