Customizer: Use theme support defaults for background images.

props philipjohn.
fixes #28586.
Built from https://develop.svn.wordpress.org/trunk@29103


git-svn-id: http://core.svn.wordpress.org/trunk@28889 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Media Grid, for audio files:

* Show `artist` and `album` fields in the Edit Attachment modal
* Sync their values on `change`

See #28839.

Built from https://develop.svn.wordpress.org/trunk@29104


git-svn-id: http://core.svn.wordpress.org/trunk@28890 1a063a9b-81f0-0310-95a4-ce76da25c4cd