Ensure that viewWillAppear can always compute CSS properties

[jsmecham]

There are times when addSubview is called on a view that is not currently in the DOM, making it impossible to perform layout or calculate CSS on the view in viewWillAppear or viewDidAppear.

We need to be aware of the cases where addSubview will not actually be adding the view to the DOM just yet and not call the callbacks until the view is added to the DOM.

For an example of where this condition exists, take a look at Aphid.UI.ListView. The ListViewItems are added to the ListView instance before the ListView is added to the DOM.