Improve handling of control priorities

[westonruter]

There needs to be a way to more cleanly maintain and extend the priorities for the controls that appear in a post section. Instead of the priorities being incremented by 10 and separated out across various methods, there could be a more central object that documents the priorities of all of the core controls, which would make it easier to maintain. Also, as there is the ability to override the labels for controls by adding special labels when calling register_post_type(), so too should there be a way to pass the priorities for the controls that appear in the section. The priorities for post field (post_date, post_content, etc) and postmeta (_wp_page_template and _thumbnail_id) controls should both be able to be managed in this way.