Twenty Thirteen: further performance for functions.js scroll event callback, props obenland. See #23875.

git-svn-id: http://core.svn.wordpress.org/trunk@24005 1a063a9b-81f0-0310-95a4-ce76da25c4cd
New Post Format UI chooser treatment in response to testing.

* Bigger icons
* Format description under each
* UI hides after choice
* Saving without choosing implies "Standard"
* Helpful text, with "Change format" link

see #24046. props lessbloat, wonderboymusic.

git-svn-id: http://core.svn.wordpress.org/trunk@24006 1a063a9b-81f0-0310-95a4-ce76da25c4cd
merged with master
Merge remote-tracking branch 'upstream/master'
Merge remote-tracking branch 'upstream/master'
Merge remote-tracking branch 'upstream/master'
Merge remote-tracking branch 'upstream/master'
Merge remote-tracking branch 'upstream/master'
Merge remote-tracking branch 'upstream/master'
Merge remote-tracking branch 'upstream/master'
Revert "Merge remote-tracking branch 'upstream/master'"

This reverts commit 2a831f4f6e32fe69d85f49e3d9d75e48be3f5cf4, reversing
changes made to c2ef2f85a24816ed67c996fa79ebe5c8e453ccb2.
Merge branch 'master' into post-formats/23965