[NUI] Add forward and backward direction type (#3932)

[NUI] Add forward and backward direction type (#3932)

Now, You can get Forward type by pressing Tab key and Backward type by pressing Shift-Tab key.

public override View GetNextFocusableView(View currentFocusedView, View.FocusDirection direction, bool loopEnabled)
{
 if (direction == View.FocusDirection.Forward)
 {
  // Tab
 }
 else if (direction == View.FocusDirection.Backward)
 {
  // Shift-Tab
 }
}