Implement saving chart to image and XPS


Charts can currently be saved to XML, but this can only be loaded in the HeadLinx app. Implement saving to image (PNG, JPG) and XPS document so it can be viewed outside of HeadLinx.
Note: It would be really nice to support saving to PDF, but there isn't a WPF to PDF writer out of the box in the .NET framework. If someone finds a suitable 3rd party library, we could plug it in.