Turn the "template" metabox functionality for pages into a capability for all CPTs

[lgladdy]

Currently, wp-admin/includes/meta-boxes.php has the following code, which determines if the "template" meta box is shown for in the admin:

if ( 'page' == $post->post_type && 0 != count( get_page_templates( $post ) ) ) {

But, custom post types should be able to use this templating functionality too. I'm sure it would need more than just patching meta-boxes.php to enable it, but it would be awesome if this was turned into a registerable capability of custom post types.

[SergeyBiryukov]

Duplicate of #18375.