Use post type label in post selector instead of post type slug

Post type slugs are not always as clean as "post" or "page" and ideally wouldn't be presented to the user. Instead of using the slug, we should use the `singular_name` post type label. See above screenshot where a post is presented in the custom post type `demo-post-type`.

Bonus points: it would be nice if the post type were on its own line, not in parentheses. Amongst the reasons, post titles may have parens, and that looks confusing. Here's an example of what it might look like, with a border separating them for additional clarity.