[NUI] Allows you to set the border to the top or bottom.

[NUI] Allows you to set the border to the top or bottom.

If you return true after overriding CreateTopBorderView(), the border UI is displayed at the top.
If you return true after overriding CreateBottomBorderView(), the border UI is displayed at the bottom.

```c#
public override bool CreateTopBorderView(View topView)
{
  // Decorate the top view
  return true;
}

public override bool CreateBottomBorderView(View bottomView)
{
  // Decorate the bottom view
  return true;
}
```