Add source file license notices

https://bugzilla.gnome.org/show_bug.cgi?id=700776
player: Has next and previous track should be false if there's no playlist

This prevents a crash when there is no selected playlist yet and an MPRIS
client tries to get properties of org.mpris.MediaPlayer2.Player interface.

https://bugzilla.gnome.org/show_bug.cgi?id=705855