Make sure `data.meta` always exists when rendering tracklists. Use `$theme_width` when rendering media tag in playlist template instead of recalculating.

Props helen.


Built from https://develop.svn.wordpress.org/trunk@27269


git-svn-id: http://core.svn.wordpress.org/trunk@27125 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Set wpActiveEditor to the id of the first instance on init, props kovshenin, fixes #27210.
Built from https://develop.svn.wordpress.org/trunk@27270


git-svn-id: http://core.svn.wordpress.org/trunk@27126 1a063a9b-81f0-0310-95a4-ce76da25c4cd