first proof of concept concerning playlists
- fixed typo queue
- press insert to add stuff to the que when you press enter it will add
the currently selected song to the queue and play the list from
beginning
- when screen-songs gets called it will check the list and if there are
still songs waiting it will start the sing-screen again, this way you
can still enter highscores for songs while using a playlist.
things to do:
- playlist class still uses regular pointer should be boost-managed
pointer and the class should be a singleton regarding to the embedded-
- after each song the playlist should display all numbers in queue on
screen for 10 seconds (this requires me to study the screen classes)
Merge branch 'playlist' of https://github.com/performous/performous into playlist

Conflicts:
	game/playlist.cc
	game/playlist.hh
	game/screen_songs.cc
	game/screen_songs.hh