[WIP] Single instance feature

- [x] add/use SingleInstance class inspired by [this](http://elegantcode.com/2011/03/02/wpf-advanced-jump-lists-using-a-single-instance-application/)
- [x] introduce ProcessCommandLineArgs to handle command line arguments
- [x] handle command line arguments (add and play media file(s))
- [x] scroll to first playable added media file
- [x] scroll only via command line added files