Video editing in the media modal:

* Add a state: `Add Subititles`
* Add `text/vtt` to the list of allowed mime-types, files end in `.vtt`. `.srt` files are served as `text/plain`.
* The content body of a video shortcode should be used for adding `<track>` elements only. This happens dynamically in the modal. If added by hand, they can still be parsed and managed.

See #27016.


Built from https://develop.svn.wordpress.org/trunk@27481


git-svn-id: http://core.svn.wordpress.org/trunk@27325 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Admin Menu: Set default value of `$icon_url` in `add_menu_page()` to `dashicons-admin-generic`, fixes #26630.
Built from https://develop.svn.wordpress.org/trunk@27482


git-svn-id: http://core.svn.wordpress.org/trunk@27326 1a063a9b-81f0-0310-95a4-ce76da25c4cd