Twenty Thirteen: take full advantage of new structured post format template functions, see r23819.

Extract first video or image from Video and Image post format posts, and place the item above the title within its own HTML wrapper with `the_video()` and `the_image()'. Also, remove the item from the post content with `the_extra_content()`.

This allows us to meet the design requirements for Twenty Thirteen with minimal changes to the theme template files. Fixes #23620.

git-svn-id: http://core.svn.wordpress.org/trunk@23820 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Add some filters to the image extraction-from-post bits so people with
existing postmeta-based flows can hook in and make magic happen.

props wonderboymusic. fixes #22960.

git-svn-id: http://core.svn.wordpress.org/trunk@23821 1a063a9b-81f0-0310-95a4-ce76da25c4cd