Edit screen UI for post formats: a first run for functionality.

* 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://core.svn.wordpress.org/trunk@23449 1a063a9b-81f0-0310-95a4-ce76da25c4cd