Add support for multimedia keys

https://bugzilla.gnome.org/show_bug.cgi?id=702879
Avoid to play a song without a selected playlist

https://bugzilla.gnome.org/show_bug.cgi?id=702879
Fix Prev/Next when the song is Paused

https://bugzilla.gnome.org/show_bug.cgi?id=702879
Avoid crash using Previous/Next (multimedia keys)

Only call _get{Previous,Next}Track if there is a previous/next track.
We didn't need to check it before because the Previous/Next buttons are
disabled in the first/last playlist's track. Now, with the Multimedia
keys we need to force the same behavior.

https://bugzilla.gnome.org/show_bug.cgi?id=702879
tests: Avoid assignment to undeclared variable

https://bugzilla.gnome.org/show_bug.cgi?id=702900