[NUI] Add method to DisposeRecursively. + Fix DisposeTest demo works

[NUI] Add method to DisposeRecursively. + Fix DisposeTest demo works

Previous Container.cs can dispose recursively only if
it is created by xaml. (DisposeIncludeChildren)
And also, that API only for internal.

Now we add some useful API that user can dispose all
connected children recursively by one button.

```
View a = new View();
View b = new View();

a.Add(b);
a.DisposeRecursively();

// Now b is disposed.
```

Signed-off-by: Eunki Hong <h.pichulia@gmail.com>