Introduce get_page_template_slug( $id = null ) to return a page's template...

Introduce get_page_template_slug( $id = null ) to return a page's template (like "showcase.php"). Returns false if post ID is not a page, and an empty string for the default page template. Use the function across core. props billerickson for initial patch. fixes #18750.



git-svn-id: https://core.svn.wordpress.org/trunk@20075 1a063a9b-81f0-0310-95a4-ce76da25c4cd