[NUI] Add InterceptKeyEvent

[NUI] Add InterceptKeyEvent

Intercepts KeyEvents in the window before dispatching KeyEvents to the View.
If a KeyEvent is consumed, no KeyEvent is delivered to the View.

```c#
Window win = NUIApplication.GetDefaultWindow();
win.InterceptKeyEvent += OnInterceptKeyEvent;

private void OnInterceptKeyEvent(object sender, Window.KeyEventArgs e)
{
   return false;   //If it returns true, other views and windows do not receive KeyEvents.
}
```