Metadata for audio and video files:

* Make attachment metadata for audio files editable by providing a metabox on the Edit Media page
* Standardize on using the attachment title everywhere
* Label the Caption and Description fields for audio and video appropriately
* Make the playlist Underscore templates more straightforward

See #27574.



git-svn-id: https://develop.svn.wordpress.org/trunk@27862 602fd350-edb4-49c9-b593-d223f7449a82