Show default audio icon in playlists when no image is present for a track

[wonderboymusic]

I went back to one of my old sites with audio files using an old plugin to play them. I made a playlist. The tracks lacked ID3 tags and thus had no images or metadata, just the title of the track (the basename of the file). The currently playing track looked sad and terrible.

[wonderboymusic]

In 27735:

Pass the default mime-type icon as the image for a track when a featured image is not selected for an item in a playlist. Image display in general can be toggled on/off in the modal via Playlist Settings.

Fixes #27525.

[wonderboymusic]

In 27742:

Make the CSS selector for current playlist item <img> more specific so the width is properly constrained.

Hide the <audio> element in the playlist to prevent a flicker when loading - MEjs hides the audio element anyway.

See #27525.