add SingleInstance class

inspired by
http://elegantcode.com/2011/03/02/wpf-advanced-jump-lists-using-a-single-instance-application/
use SingleInstance and introduce ProcessCommandLineArgs (nyf)
fix activating/showing single instance

use Activate/Focus and not SetForegroundWindow
activating/focusing works now with these changes
handle command line arguments
add ListBoxAutoScrollBehavior

just scroll to the selected media file
fix first start to handle the command line
handle command line after loaded playlist
scroll only item into view via command line
Merge pull request #7 from punker76/SingleInstance-feature

[WIP] Single instance feature