player: No next/previous track if there is no playlist

Fixes Play on MPRIS when there is no playlist.

https://bugzilla.gnome.org/show_bug.cgi?id=708103
mpris: Use filter instead of the view model

https://bugzilla.gnome.org/show_bug.cgi?id=708103
Add StartupNotify to desktop file

https://bugzilla.gnome.org/show_bug.cgi?id=707824