Accessibility: Notification for screen changes

For VO and probably TB there are notifications to say "screen contents changed".
Implementing that in for example the qquick page stack is required.
Alternatively we could send the notification when accessibles are created/destroyed.

https://developer.apple.com/library/ios/documentation/UIKit/Reference/UIKitFunctionReference/Reference/reference.html#//apple_ref/c/func/UIAccessibilityPostNotification