* Adds a very basic tabbed interface for selecting a post format (requires JS).
* Extra fields, which are post meta, are shown/hidden based on the selected format.
* Introduce a helper function for retrieving formats-specific metadata: `get_post_format_meta()`.
* Image selection uses the media modal, although without filtering or from URL support at the moment.
props rachelbaker, wonderboymusic, aaroncampbell, helen. see #19570.
git-svn-id: https://develop.svn.wordpress.org/trunk@23449 602fd350-edb4-49c9-b593-d223f7449a82
