Autoload external audio files only if there's at least a video track
(which is not coverart pseudo-video).
Enable external audio file autoloading by default. Now that we actively
avoid doing stupid things like loading an external audio file for an
audio-only file, this should be fine.
Additionally, don't autoload subtitles if a subtitle is played.
Although you currently can't play subtitles without audio or video,
it's disturbing and stupid that the player might load subtitle files
with different extension and then fail.