Added basic Gapless Playback support for mpv

Using the mpv playlist API I was able to add basic gapless playback support to the mpv player.

I changed SublimeMusicApp to notify the PlayerManager and the Player when the next song in the current play queue has been downloaded. The Player will then add it to mpv's internal playlist after the current song. This results in mpv's gapless playback support handling the transition between the current song and the next one.

I can split the 7643a770 commit into it's own MR if that makes more sense.

Closes #73 (closed)