fixed navigation button problems

I also removed the insert-button since the menu can now be used for
adding songs.
TODO:
- adding a few menu-options
- adding a function to return the current playlist as string /
stringstream
- adding surface to display list after each song
- add function to generate a menu for editing the playlist between songs
(playlist is displayed for 10 seconds after each song, pushing a button
in these 10 seconds will popup the edit-menu)

also next friday is karaoke at our student society, I´ll try to finish
the above tasks by then so i can test the implementation properly.