Allow display of custom visibility settings

[mitchoyoshitaka]

Right now the post_status setting on a post is translated into a "visibility" value when the edit post screen is set up. There is no way to interact with custom post_status values.

A first step towards this would be to allow custom mappings of post_status (and other information) to custom visibility values (patch supplied). Saving custom values can be handled using the wp_insert_post_data filter.

Ultimately, a way to swap out the div#visibility printing code would be ideal, but for my current application I could just as well use some after-load javascript to make this happen, given the correct visibility value being set in #hidden-post-visibility.

Duplicate #12706, #12567.