Toggle 'Slide Options' metabox when post category is checked on a
'Post' admin screen.

 When the slider's featured category is checked in the "Add New Post"
 or "Edit Post" admin page, some inline javascript will either show or
 hide the "Slide Options" and "Video Options" metaboxes.

 This only takes effect if the slider is set to use a post category.
Only register 'features' post type if slider is in post type mode.

Basically, redo the logic in commit b2a13d73c2.
Follow WP whitespace and braces convention.