Add functions for generating metadata for video and audio, using the
ID3 library. Also allows themes/plugins to add thumbnail support
to these media types. Think stuff like album art, movie covers, and
video freeze-frames.

props wonderboymusic. fixes #23673

git-svn-id: https://core.svn.wordpress.org/trunk@23766 1a063a9b-81f0-0310-95a4-ce76da25c4cd