[NUI] BackKeyEvent Proptotype (#1824)

[NUI] BackKeyEvent Proptotype (#1824)

View can subscribe back key event using BackKeyEvent event handler.

When user adds BackKeyEvent callback to View, view is listed as Subscriber of BackKeyManager.

After that, if back key is comming,
BackKeyManager will find top view among back key subscribers which has Visibility and IsOnWidnow property are true.

The top view will emit its BackKeyEvent.

*** Notice ***

It is based on Window KeyEvent so make sure you don't use Window KeyEvent and View KeyEvent.

After binding backkey event from DALi, user can use Window/View KeyEvent
with BackKeyEvent.