Utilize new temp hooks to make detection of sidebars in preview more robust

[westonruter]

See:
http://core.trac.wordpress.org/ticket/25368
http://core.trac.wordpress.org/changeset/25580

With these temp hooks, we can now reliably detect if a sidebar is used on a template, whereas before it would fail if the sidebars were empty.

[westonruter]

Note that the WordPress 3.7 release removed the temporary hooks (see /src/wp-includes/widgets.php@r25878), but they remain in trunk.