We need to have access to UI elements from script

Currently, it is very difficult and frustrating to customize ui elements on installer pages. There is no consistency at all.

We should be able to easily access ui element properties (such as visible, disabled, text, etc...) on every installer page.

Presently, if I do: 

Controller.prototype.IntroductionPageCallback = function()
{

var errorLabel = widget.findChild("ErrorLabel");

console.log(errorLabel);

}

it returns a valid handle.

However, if I do:

Controller.prototype.FinishedPageCallback = function()
{

var mybutton = widget.findChild("FinishButton");

console.log(mybutton);

}

it returns null. This should not happen.