Twenty Twelve: change form input selectors to only reset non-textual input...

Twenty Twelve: change form input selectors to only reset non-textual input types, allowing for broader support for newer HTML5 input types. Also add better {{{button}}} support. Fixes #20579.


git-svn-id: https://core.svn.wordpress.org/trunk@22899 1a063a9b-81f0-0310-95a4-ce76da25c4cd