Improve body class for page templates.

* Convert slash to dash too.
* Add sub-folder name as a separate body class.
* New classes are additional classes, old syntax will persists for BC.

With this `/page-templates/full-width.php` will produce `page-template`, `page-template-page-templates`, `page-template-full-width` and `page-template-page-templatesfull-width-php`.

props obenland, Caspie, donutz.
fixes #23470.

git-svn-id: https://develop.svn.wordpress.org/trunk@30100 602fd350-edb4-49c9-b593-d223f7449a82