preview button shown on post screen with non-public custom post types

[intoxstudio]

Registering a custom post type with the following arguments

$args = array(
'public' => false,
'show_ui' => true
);

means that the post type shouldn't be public but still shown in the administration (obviously). Therefore it makes no sense that a preview button is visible on the post screen, as it will lead to nothing (404); ie. the use case is nonexistent.

This patch checks if a post type is public before adding the preview button to the post screen.

[SergeyBiryukov]

Related: #17609