Skip mouse events of unloaded controls

[Khaos66]

Somehow mouse events are fired for unloaded tabitems resulting in a NullReferenceException

at AvalonDock.Controls.TransformExtensions.PointToScreenDPI(Visual visual, Point pt)
   at AvalonDock.Controls.TransformExtensions.GetScreenArea(FrameworkElement element)
   at AvalonDock.Controls.LayoutDocumentTabItem.UpdateDragDetails()
   at AvalonDock.Controls.LayoutDocumentTabItem.OnMouseMove(MouseEventArgs e)
   at System.Windows.RoutedEventArgs.InvokeHandler(Delegate handler, Object target)
...

This fixes the exception