Edited Issue: ZoomBox needs width/Height to display something [21017]

Using the following code won't show the content in the ZoomBox:
```
<xctk:Zoombox Name="_ZoomBox">
<ItemsControl Name="_ItemsControl" />
</xctk:Zoombox>
```
To make it work, we need to set a size on the ZoomBox :
```
<xctk:Zoombox Name="_ZoomBox" Width="400" Height="400">
<ItemsControl Name="_ItemsControl" />
</xctk:Zoombox>
```

ZoomBox should set its size based on his content.