Addressing MR comments

- Inverted conditional in PlayerManager.play_media to flatten code some
- Simplified check for gapless_playback in MPVPlayer
- Removed left-over code in SublimeMusicApp.on_track_end
- Changed song-details lookups for gapless playback to always defer
- Updated on_song_download_complete to use new next_song_index property
3 jobs for gapless-playback-mpv in 3 minutes and 40 seconds (queued for 1 second)