Multiple items deselected in ListView and other subclasses of ListBox 

Hi, first of all, many thanks for taking over this project. Here's a little pull request to fix a small bug: whenever I tried to select multiple items from a ListView and drag them, all the items would be deselected, except the single item I actually started dragging.

This was due to a bug in `ItemsControlExtensions.GetSelectedItems()`:

```csharp
if (itemsControl.GetType().IsAssignableFrom(typeof(ListBox)))
{
    var listBox = (ListBox)itemsControl;
    ...
```

This is too strict - the IsAssignableFromType() fails to handle ListView and other subclasses of ListBox. My patch simply changes it to an is operator instead.