API for adding settings to default settings pages

[ryan]

Plugins should be able to integrate new settings and new sections of settings into the default settings pages via API.

add_settings_section(), add_settings_field

Patch implements add_settings_section() and add_settings_field() and changes options-writing.php to make use of them.

Test code

(In [8855]) add_settings_section() and add_settings_field(). see #7682

Anybody confirm this is functional for plugins in 2.7? Doesn't seem to work or maybe I'm missing something.

function myex_conf()
{
  add_settings_section('myex_settings','My Example Settings','myex_settings','reading');
}
add_action('admin_init','myex_conf');

function myex_settings()
{
  echo "foo";
}

The API works fine for writing, discussion, etc - but does not work for reading. Reopening.

(In [10003]) do_settings_sections('reading'). fixes #7682

Related: #7092